"a written or spoken agreement, especially one concerning employment, sales, or tenancy, that is intended to be enforceable by law."
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.