It is not about repo'ing Twitter. The collateral on the loan is Tesla stock. If Musk is forced to sell stock to repay the loan, he will have to pay taxes on them. Big time. Because the whole idea of loans with stock as collateral is that money from loans is not taxed. So he never pays taxes, for many years. And then you regularly refinance a loan with an even bigger loan until you die - or your loan gets too big for your collateral.
Imagine what will happen if he has to sell stock to repay way beyond 100B in loans (this is not just Twitter, he finances his whole live for years this way!), and no ways to really evade taxes on this.