There are some exit clauses (like Musk not getting financing from his banks) but they don't apply here. Specifically, there's no exit clause for the bot percentage.
> If it was a promise it could be written on a hallmark card.
Are you just trying to change the meaning of the word promise? Most people would agree that if you break a promise you pay a price - it might be a social one in case of a promise between friends. Or it might be a financial price in case of a written & signed promise, a.k.a a contract.
Musk is actually obligated to buy the company at this point. It is unlikely the courts will fully attempt to force such a large purchase. But the penalty decided could be much higher than $1B.