Just over year ago, Twitter shares traded at about $70 per share. Musk's offer is about 25% lower than last years peak. There's no reason to assume Twitter shares can't reach the same levels ever again, nothing much has changed fundamentally over a year.
Musk's offer is a cash only offer. $43 billion is a lot of cash. That's a lot of Tesla shares to sell, or massive loan. If Musk thinks Twitter is that valuable to take such risks, current major investors will probably want to unlock that value themselves.
I think the takeover offer is just as believable as last week's announcement that Musk was joining the Twitter board.