"If the LLM trained on GPL-derived code, what does that mean for the end result?"

Either it means nothing at all, or it means that a substantial portion of the code produced at some of the world's most valuable companies over the last two and a half years is GPL.

Yeah you could argue that, but it's a messy argument. I'm wiling to imagine that they hadn't used AI to do the rewrite to Rust. It wouldn't have been all that hard to do with plain old syntax tree transformation (my expertise). The trickier question for me is the ownership at the boundary where prompts were turned into Zig code. That is not a process that could have been done with syntax tree transformation: creative freedom was given to the model as to how to do the work at that stage, and so that is the stage at which I'm least sure they could legally defend their claim to ownership.