This is shockingly true. Most newer FE devs I have encountered are mostly trained on the popular frameworks and lack understanding of the underlying fundamentals, e.g., they only know TypeScript + SCSS and some smattering of HTML but more often know whatever templating engine and MVC(ish) backend the framework uses. It’s really helpful to understand what the browser is actually doing and all the “stuff” the framework spits out on the other end.

Modern JS/TS devs probably not, but I wouldn't even call someone a "frontend dev" if you don't know HTML, kind of being a infrastructure engineer and not knowing how any OSes work.