I'm just a well-read interested amateur, not a lawyer, let alone an American contract lawyer.

Leonard was a case where Pepsi advertised, basically, that they'd sell you a Harrier Jet for $700,000. Leonard sent them a cheque for that amount and tried to get them to honour the deal. There was nothing explicit in the ad that made it a joke, but Pepsi refused, and ultimately won the lawsuit because it was absurd. Likewise, in my opinion, giving away a multibillion dollar company on the basis of three tweets, one of which is describing the possibility of a mysterious death, and another which is literally just "Ok" is very similar to that situation, in that it's played straight, but is obviously a joke because of the content.

You're right that a lawsuit could be brought. I suspect it would even pass summary judgment, because whether or not it's a joke would be a finding of fact, not a simple finding of law. And I don't know what's hypothetical Musk Estate would do, but if it were Elon himself in charge there's a good chance you're right, he'd try to settle it. Not because he's afraid of losing or concerned about the cost of the lawsuit, but for the same reason he hurried to buy Twitter when he did: to avoid going through discovery. He obviously doesn't want details of his finances made public, for whatever reason. And his estate might very well inherit that shyness.

But all that would rely on Mr Beast being stupid enough to press his claim in the first place. I only know about him third-hand as a famous YouTuber with a history of doing some rather silly stunts, but surely this would be beyond him. The case might make for good Content™, but entering into it would be very expensive with almost zero chance of proper success and nowhere near a guarantee of even a favourable settlement.