At the margin means that both languages, or frameworks or whatever, are reasonably appropriate for the task at hand. If you are writing firmware for a robot, then the LLM will be less helpful, and a language such as Python or JS which the LLM is good at is useless.
But Thomas's point is that arguing that LLMs are not useful for all languages is not the same as saying that are not useful for any language.
If you believe that LLM competencies are not actually becoming drivers in what web frameworks people are using, for example, you need to open your eyes and recognize what is happening instead of what you think should be happening.
(I write this as someone who prefers SvelteJS over React - but LLM's React output is much better. This has become kind of an issue over the last few years.)
Taking your react example, then if we we're a couple years ahead on LLMs, jQuery might now be the preferred tool due to AI adoption through consumption.
You can apply this to other fields too. It's quite possible that AIs will make movies, but the only reliably well produced ones will be superhero movies... (I'm exaggerating for effect)
Could AI be the next Cavendish banana? I'm probably being a bit silly though...
I'd argue that the Web development world has been choosing tooling based largely on popularity for like at least a decade now. I can't see how tooling selection could possibly get any worse for that section of the profession.
The argument is that we lose this diversity as more people rely on AI and choose what AI prefers.
Of course you could disagree with my prediction and that these big tech companies are going to build MASSIVE gpu farms the size of the Tesla Gigafactory which can run godlike AI where nobody can compete, but if we get to that point I feel like we will have bigger problems than "AI react code is better than AI solidjs code"
You misunderstand me. It's not incompatible for a culture to choose options based largely on popularity (rather than other properties that one would expect to be more salient when making a highly-technical choice), and for there to also be many options to choose from.