The person you're responding to wrote about "a semantic web", not the Semantic Web. Ordinary class names, done properly, add semantics to Web pages, and this is what went on for years without Web developers necessarily trying very hard or even knowing that's what they doing—until people started abusing the medium and writing CSS compilers/automanglers and then, later, stuff like Tailwind.

Did developers adopt the ideas? Mostly no. But were they actually still doing it, despite not having bought in to the philosophy? Yes.

"No adoption" is certainly superlative. That existing sites rarely go through top-down redesigns is part of the slow adoption, though. Are there really designers who have been exposed to the idea of semantic design and yet are still making designs with <div> soup instead of <main>, <header>, etc? I would certainly hope not.