From memory I think it still had to go to trial and he had a good chance of weaseling his way out of it. Apparently there was a lot of stuff that would have come out in discovery so what ever was in there was bad enough that he preferred to take a really bad deal than let people find out what ever he’s been up to. For money grub musk that must have been very bad to lose money in favour of maintaining his privacy.

Borrowing with stocks as collateral should be taxed at the capital gains rate

He did sell the stock, like $23 billion worth. He entered the agreement to buy Twitter to show that he had another use for that cash, so that Tesla investors didn't get spooked and sell off when they see the biggest shareholder selling (along with the downward price pressure that comes from selling a significant percentage of a company's stock).

There was some speculation at the time that he entered the agreement with Twitter with no intention to close, just to cover his desire to cash out of Tesla at its high. Then the courts actually held him to that.