Huh, I've been assuming the opposite: better to use React even if you don't need it, because of its prevalence in the training data. Is it not the case that LLMs are better at standard stacks like that than custom JS?

Hard to say for sure. I've been finding that frontier LLMs write very good code when I tell them "vanilla JS, no React" - in that their code matches my personal taste at least - but that's hardly a robust benchmark.