But as the richest man in the world, he could cover their stakes.

As previously stated, no. He couldn’t. He needed their money just to acquire the company, he did not have enough liquid assets himself to make the purchase and sold a lot of his stock in Tesla beyond just the equity shares. If Twitter fails he’ll be on the losing end financially for once, so he needs it to be successful. The courts did the world a favor and forced a purchase by actually upholding the law.