That's not what I'm asking about. This thought experiment is meant to argue that not all knowledge is physical, right? Set aside any notion of what those terms

That's not what I'm asking about. This thought experiment is meant to argue that not all knowledge is physical, right? Set aside any notion of what those terms mean and consider it in the abstract. It sets out to argue that "Not all X has property Y", and then asks us to consider a hypothetical situation with the premises "Assume that Mary has all X with Y" and "Assume that later she finds more X". The thought experiment doesn't support the claim that not all knowledge is physical, it requires it - i.e. the definition of begging the question. Perhaps the point here is that "Mary gains new knowledge" is supposed to be so manifestly obvious that it doesn't need to be argued? If so, this seems far from obvious to me.