I think this is reasonable but incorrect. Tailwind didn't say "our conversion rates went down." They said the traffic to their docs site went down.

Their funnel depended on SEO, and LLMs took SEO out of the equation. What you said about the value of pre-built components might also be true but can't be determined from a situation where consumers suddenly all stopped learning that the pre-built components exist. Occam's Razor says it's the funnel.