Google openly train stuff based on your email, they used is specifically to train Smart Compose, but maybe other stuff too. He likely uses multiple Google products. Draft papers in Google Drive perhaps?

These LLM models are essentially trying to produce material that sounds correct, perhaps the hypothesis was a relatively obvious question with the right domain knowledge.

Additionally, he may not have been the first to ask the question. It's entirely possible that the AI chewed up and spat out some domain knowledge from a foreign research group outside of his wheelhouse. This kind of stuff happens all the time.

I personally have accidentally reinvented things without prior knowledge of them. Many years ago in University I remember deriving a PID controller without being aware of what one was. I probably got enough clues from other people/media that were aware of them, that bridging that final gap was made easier.