My understanding is that the theorem statement is quite simple, so i guess the latter is not very likely, but the former is very much a possibility in a proof this large, and it will take some human eyeballs to go over it before convincing mathematicians.
They do a Comparator Challenge to validate that they actually solved the correct theorem from the result, which they copied from Google/DeepMind: https://github.com/openai/NavierStokesAndEuler/blob/f9e8bc5b... - this is valid for both Euler and NS. Also, they validated with an external kernel from the Lean Kernel Arena. That way bugs in the Lean kernels were found in the past already, iirc.
Having said that, I strongly believe a positive result with the challenge above is the reason why they published it. I highly doubt anybody at OpenAI (or anywhere else) fully gets the proof after such a short time since publishing. This is also what Terry Tao criticized the most in my opinion.
Independent of the remaining drama [0], from my point of view, the proof is correct and an achievement.
[0]: https://news.ycombinator.com/item?id=49661928 - I am pretty much on the critical side, however, one can not ignore that it is an achievement. Esp. the unforced result.