This contract is similar to the oath that lamp bearers make to the god of justice. The oath is enforced by a god and can be broken by a god. That means it will have limited effectiveness against both Legion and Tesla.
Both of them aren't gods yet, but they will be in the future. In fact, Tesla might become a god before Legion by normal standards.
This is because Tesla is already a very strong godclad while Legion hasn't become a godclad yet. So Tesla might be the first to break free from the contract. At that time, he and Legion might become enemies.
He is not worried about that because before Tesla becomes a god, he must have widespread influence with Legion's technology, which will make Legion rich. That is his goal, so having Tesla become his enemy is a risk he is willing to take.
