I don't think that's true anymore. I've seen a few benchmarks where svelte results in less tokens than React frameworks 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. The knowledge cutoff isn't so bad and LLMs are much better at copying the patterns in your own codebase rather than relying on baked in knowledge.

Edit: This is the benchmark. But if you suspect it's wrong it's always fun to run your own evals. https://martinalderson.com/posts/which-web-frameworks-are-mo...