I don't think that's true anymore. I've seen a few benchmarks where svelte results in less tokens to complete the task. There was a time when LLMs were awful at svelte (especially v5) due to stale knowledge but that's not the case today.
every time I see this argument I think of the fact that one of my current side projects in D with qt bindings off github - an obscure library for an obscure language - and claude had zero issues with it. I had to guide the architecture pretty heavily, but the fiddly bits of interfacing multithreaded c libraries to D's garbage collector and qt's event loop were all thanks to the LLM, and done a lot quicker than I would have.
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.
i have had similar thoughts. But i hold on to the idea that its still valuable to invent new tools and ways of doing things. other wise, why not just use html css and javascript and not use react at all?
I've had to work on a Next.js for work and much prefer Sveltekit. Excited to try this release!