He explicitly states that the AI training concerns are not about legal GPL violations but about going against his licensing intentions (and those seem very much in line with the "copyleft spirit" from what I can tell).
My take is that the LLM emergence "threatens" the whole copyleft framework in a way similar to cloud services in the past (which led to the AGPL): closed source development can extract a lot of value from copyleft projects without contributing back in any way (to neither upstream nor their own users).