In my experience 'Kohlefaser' is a pretty common technical term, I've only ever encountered 'Karbon' w.r.t. bicycles (but that just be my limited view).
The literal translation for carbon fiber would be "Kohlefaser" and that is also used quite commonly.
I admit I'm not in any field that deals with materials a lot, but OP said "Kohlefaserverstärkter Kunststoff" (which I likely wrongly called "carbon fiber" in English), is "Kohlefaser" really the same? Maybe it is and I'm just ignorant about that.
Depends. (As everything always does.) “Carbon fiber” literally means just the black threads, in English as in any language. When you say “This bicycle / fishing rod / automobile part is made of carbon fiber”, what you _actually mean_ is that it's made from carbon fiber- reinforced polymer (=“plastic”). Stuff isn't built from _just_ the fibers; the plastic needs to be there to keep them together / they're just there to reinforce the plastic (or both).
The GP (G-G-G...P?) post seemed to be using the shorter expression, but as a shorthand for the longer (as most people do) in English but not in German, which may have caused some confusion.
But it's the same in both (all) languages: The shorter expression technically means just the reinforcing fibers; the actual material used to make stuff is technically called the longer expression; but in ordinary usage most people use the shorter expression to mean the longer one.
My German-as-second-language understanding is:
Kohlenstofffaserverstärkter Kunststoff == Kohlen (literally coal but understood more broadly as any carbon) stoff (material) faser (thread) verstärkter (reinforced) Kunststoff (plastic).
I’ve heard it being called just “Carbon”, pronounced with a long “o”, frequently. Since the translation for the generic element, carbon, is “Kohlenstoff”, and that word is very commonly used, the opportunities for confusion between carbon and carbon fiber is less than it might seem.