It's rather convenient for OpenAI that user logs are de-identified before being fed into training, so they can say "there's no way for us to check if we plagiarized our user's work, because user data is private".

It's also difficult to imagine any competent AI researcher would overlook the possibility of training data leaking into the test, especially given that they know the users have been using their model to work on the same problem, and that they jumped on the problem after hearing rumors of the breakthrough.