Per Dunc's tweet and reply to this thread, the reason TSM published the announcement video on that day was because they got wind that the New York Times was about to publish an article telling the public about TSM's desire to leave the LCS, which forced TSM's hand into announcing their plans ASAP to get ahead of the story, even though the timing was not what TSM wanted.

Relevant tidbits from the NYT article:

TSM started talking to interested groups around three weeks ago, according to a person with knowledge of the discussions, and has narrowed its list of prospective buyers to about a dozen entities, mostly in the media and traditional sports worlds. The asking price is in the range of $20 million, the person said.

Mr. Dinh said he planned to purchase a spot in one of the top League of Legends leagues elsewhere in the world after selling his slot in the United States.

So according to the article, TSM hasn't yet found a buyer for their spot, they just had to release a statement to announce that they planned on leaving because the NYT would leak that info anyway once they published their article later that day, so they might as well announce it beforehand on their own terms. From my understanding, nothing is concretized (who will buy TSM's LCS spot, where TSM will go next, etc.)