Knuth has only so many productive years left on this planet and he wants to spend them all on TAOCP (and a few other projects). He had given up email by the 90s and he is incredibly disciplined on what he devotes his time to. If you want to get in touch, send him an actual letter. Last I checked, he reads paper mail once a quarter or so. He's also an utter perfectionist in his own work, in a way that few people ever achieve.

So he's curious enough to try ChatGPT. But then he's smart enough to walk away.