The merger contract provides the option to pay $1b to walk away under only limited circumstances: either the acquisition is blocked by a government, or Musk fails to get financing for the deal. The latter can't really be easily faked: if he said 'sorry, I thought I had the money, but I checked my couch cushions and it turns out I don't,' and paid the $1b to walk away, Twitter would sue and this assertion that his financing broke down would be tested by the courts.
In fact, out of those limited allowances to pay the $1b and walk away, Musk is bound by the contract to 'specific performance'. That is, he's bound to actually do it. Pay the $54.20.
A good M&A analysis of the situation is below.
https://yetanothervalueblog.substack.com/p/quick-twtr-though...