This question can only come from someone who heard about the Sicha but never read it inside. The Sicha begins by saying explicitly (p. 350) that the time has now come to say Yechi HaMelech! The Sicha ends by stating explicitly (p. 354) the Hora’a pertaining to practical action- we are to end the Avoda of Golus with the announcement of ad Mosai and Yechi HaMelech! How can one conclude from this the reverse, i.e., not to say Yechi HaMelech?!
What then does it say in the Sicha? The Rebbe says that in recent years, he has repeatedly spoken about announcing ad Mosai because it is a way of telling Hashem that we want the Golus to end. In this Sicha, for the first time, the Rebbe reveals that besides expressing our desire for Geula, the words ad Mosai actually are the coronation of Melech HaMoshiach, as are the words Yechi HaMelech [when we are asking for Geula to come, in essence we are asking for the Malchus of Melech HaMoshiach]. Implicit in the meaning of ad Mosai is the idea of Yechi HaMelech, which is another reason why the Rebbe had told us to announce ad Mosai [although this was unknown to us.] Now is the time, says the Rebbe, to start saying Yechi HaMelech as well as ad Mosai. In other words, the Rebbe is saying that not only is he telling us now to say Yechi HaMelech for Hachtaras HaMelech [coronation of the king], but throughout all the years that we have been announcing ad Mosai, these words contained within them the idea of Hachtaras HaMelech. The point is clearly “What you are being told to do now had actually begun a long time ago,” not “Don’t do it now!” The following explains this fundamental point of the Rebbe which is well known to all those who are familiar with the Rebbe’s Sichos:
In every generation, there are new ideas of Torah that are introduced that were not known before. The Rebbe explains (Likkutei Sichos, vol. 7, p. 206) thatכל מה שתלמיד ותיק עתיד לחדש ניתנה למשה מסיני. This means that innovations in Torah are not totally new, Chas VeShalom, but hidden and included in the teachings which we previously had.
However, הכל עשה יפה בעתו, there comes a time when a detail which was hidden and included in the earlier teachings becomes revealed as a separate and independent concept, and remains that way from that point on.
A case in point is the revelation of Chassidus itself. Since all of the concepts of Chassidus are found in Torah and are not something new, G-d forbid, one could think that perhaps it is not necessary to learn Chassidus. After all, the ideas are there, albeit hidden in Zohar, Tehillim, Chumash and Gemara. Maybe we should continue learning only Nigleh, as was done in the past, and inherently, the teachings of Chassidus would be learned. Of course, this makes no sense, because in that case, what was the whole point of the Baal Shem Tov and his teachings? Obviously, the innovation of the Baal Shem Tov is that the time had come to study Chassidus openly and explicitly as a separate subject of Torah.
A more specific illustration of this is found in the second volume of Tanya, Shaar HaYichud V’HaEmunah, chapter 1. The Alter Rebbe bases this entire volume on the words of the Baal Shem Tov’s interpretation of the Posuk לעולם ה’ דברך ניצב בשמים.
The Rebbe brings the question that the Baal Shem Tov’s interpretation is already found in the Midrash on Tehillim. The Rebbe answers by saying that the interpretation was unknown and the Baal Shem Tov was the one to publicize, elaborate and form the ideas of Chassidus based on the interpretation of the Midrash. This is the meaning of the words in Tanya ופירוש הבעש”ט. It does not only mean that the Baal Shem Tov explained this interpretation, but the Baal Shem Tov spread it out, i.e., publicized and developed it (Likkutei Sichos, vol. 3, p. 64).
One could therefore ask, “Why learn Shaar Ha Yichud V’HaEmunah? Let’s learn the Midrash which includes within it the ideas of Shaar HaYichud V’HaEmunah?!” Of course, this is not a question because the time has come to bring out openly, and develop separately, that which used to be hidden in the Midrash.
Another example of this is אימתי קאתי מר לכשיפוצו מעיינותך חוצה. The Sicha of Parshas Ki Saitzei (Sefer HaSichos 5748, p. 629), discusses that from the times of the Baal Shem Tov and on, people were aware that the whole purpose of teaching and spreading Chassidus is to bring Moshiach. Everyone knew that Moshiach had told the Baal Shem Tov that when his wellsprings would spread forth, Moshiach would come. But this was not emphasized. It was somewhat concealed. The emphasis was on teaching and spreading Chassidus because of the benefits which it brings in one’s personal spiritual life.
The idea of לכשיפוצו מעיינותך חוצה קאתי מר דא מלכא משיחא was seldom mentioned. When the Frierdiker Rebbe announced “LeAlter LeTeshuva, LeAlter LeGeulah,” that is when the connection was openly emphasized between Chassidus and bringing Moshiach. From that point on, this emphasis became progressively stronger and more open. The connection between spreading the well springs and bringing Moshiach was not emphasized in years past. It was hidden and included in the other reasons given for learning Chassidus. Should we therefore not emphasize it now?! It is understood that we have progressed to the point that from now on, this should be emphasized openly and explicitly.
This reminds one of the stories of the Alter Rebbe. Before he became Rebbe, his in-laws gave him a very difficult time because he lived according to the teachings of Chassidus. They connected him and frequently demanded the Rebbetzin to divorce him.
There is a particularly famous story with his father-in-law. He did not approve of the way the Alter Rebbe davened late. One Shabbos, he hid all the wine and Bechers so the Alter Rebbe would not be able to make Kiddush. When the Alter Rebbe finished davening, he could not find any wine, but he found Mashke. He also could not find a Becher, but he found a soup bowl. So, he made Kiddush on the Mashke using the soup bowl. After Kiddush, he began to learn. His father-in-law taunted his daughter commenting that the Alter Rebbe had been unable to make Kiddush. She explained to him how, in fact, he had made Kiddush. When her father replied that the Alter Rebbe must therefore be sleeping under a bed somewhere, she showed him that the Alter Rebbe was sitting and learning- his face flaming red, completely oblivious to everything going on around him, her father was impressed but still did not change his ways.
When the Alter Rebbe later became Rebbe, his father-in-law was no longer alive. His mother-in-law, still alive, apologized and begged the Alter Rebbe and the Rebbetzin to move into her home so she could support them and make up for the past mistreatment. But he refused, explaining himself with the following analogy. Before a baby is born, it is inside his mother. It learns Torah with an angel and lacks nothing. But once the baby is born and starts to become independent, it does not want to return to its former place despite the positive aspects, because it grew out of that space. The Alter Rebbe had grown out of that space/stage and could not go back to live in her home.
Likewise, any matter in Chassidus which for years was hidden and then becomes revealed means that it has reached a new stage. It has been “born” and from that point on, has a life of its own. The Rebbe says that in the past, we did not say Yechi HaMelech explicitly because it was included in ad Mosai. Now the time has come to say Yechi HaMelech in an open way. It therefore directly opposes the whole point of the Sicha to go “back” and Say Yechi HaMelech via ad Mosai.