> Human games are still important for the advancement of chess

Are they? Why?

For a human vs human game sure but at the very top level? No of course not because it's all done by AI.

> Only if it is in the training data.

This is trivially not true, as how has AI managed to become better than humans if the knowledge of how to do so never existed in the training data.

We are well past AI can't do X unless X is in the training data. If your claim were true then AI could never surpass human expertise in any field because by definition all the available training data will at best be at the current human frontier and not beyond.