We can only go off their word unfortunately and they say no formal math. so I assume it's being eval'd by a verifier model instead of a formal system. There's actually some hints of this b/c geometry in Lean is not that well developed so unless they also built their own system it's hard to do it formally (though their P2 proof is by coordinate bash (computation by algebra instead of geometric construction) so it's hard to tell.
We’re talking about Sam Altman’s company here. The same company that started out as a non profit claiming they wanted to better the world.
Suggesting they should be given the benefit of the doubt is dishonest at this point.