The $1B isn't sufficient, though. He would be subject to some serious scrutiny by the SEC.
The $55B isn't all other people's money. He sold a ton of Tesla stock to put up his share (about half, IIRC). That alone would draw SEC ire, manipulating the Tesla stock price under apparently false pretenses.
I suspect that this is the reason his lawyers finally got through to him: go through with the deal or be in very big trouble. He will spend a lot of his own money on it, but he does at least get a major social media corporation for that money.
Whether it's worth it, either in future earnings or in the power he gets from it... I have no idea. Probably not, seeing how poorly considered the whole thing has been so far. But then I don't see the appeal of Twitter at all, so I can't predict what will happen to it in the future.