Because the models are trained on hundreds of billions of user conversations, across more than a billion different humans. The conversations are anonymized and not easily traceable back to a specific user.

It's unknowable and not possible to prove if any one specific conversation contained the insights for solving Navier–Stokes.

We also don't know if the authors unintentionally provided data to OpenAI through alternate means, such as via alternate accounts or model feedback queries.

Their base model must have been trained with hundreds of trillions of tokens several months ahead, at this point of time, it is impossible to rule out the possibility the model had seen that session at one point of time, and it probably did, without any OpenAI personnels actually know about it.

Because OpenAI says so, obviously!