In my experience LLMs write mediocre React code: it works, but is the most naive implementation possible for a given task. It needs to be bullied extensively to write performant React that considers prop stability, does updates in event handlers instead of convoluted useEffect chains, etc. I haven't tried Svelte in a while but "lots of React in training data" doesn't feel like a huge boon to me.

Tells you something about the average React code