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.
> Ordinary class names, done properly, add semantics to Web pages
CSS classes added semantics, really? Assuming that is true, who exactly was this visible to? Definitely not any reasonable definition of society as the GP claimed.
> Classes are a markup concept. The class attribute predates CSS; CSS's selector language lets you target "classified" content, but it's not like classes were invented for CSS.
"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.
Did developers adopt the ideas? Mostly no. But were they actually still doing it, despite not having bought in to the philosophy? Yes.