Why would he want to be purchased by twitter? I assume he still has equity there, so it wouldn't make too much sense to keep all the eggs in one basket. Increasing his equity share at twitter would be somewhat meaningless, since I'm sure he could always go back to twitter and continue working.
If he sells Medium to Twitter for stock and cash and the cash portion of his stake in Medium is worth more than what it would add to the value of his stake in Twitter, or he is for some reason unable to cash out some of his Twitter stake, then he might have good financial reason to sell to Twitter.
However, I agree that is unlikely to be his goal since he has taken enough off the table in Twitter fundraising rounds that he likely wouldn't need to sell just for the money and re-entry into Twittrr, which would likely be the purpose of such an acquisition.