That, is the big question!

It's not a 100.0% guarantee, but the same goes for most diplomatic promises (especially when one administration of a country makes a promise on behalf of future administrations). Yet diplomacy still works much better than nothing!

It may implicitly be a promise to try really really really hard to prevent the other race participants from regretting it, rather than a promise to algorithmically guarantee it above all else. A lots of promises in real life are like that, e.g. when you promise your fiancé(e) you'll always love him/her.

Hopefully this question can be discussed in greater depth.

PS:

Promises made by AI researchers and AI labs help reduce the race within a country (e.g. the US). Reducing the race between countries is best done by promises from government leaders.

But even these leaders are far more likely to promise, if the promise has already been normalized by people below them—especially people in AI labs. Even if government leaders don't make the promises, the AI labs' promises could still meaningfully influence the AI labs in other countries.