Except that’s not what we mean when we talk about ghosts. Ghosts are meant to be actual beings with an actual existence, if very different from living beings.

The concept of ghosts exist (as does for all things for which we have words). Some people do believe ghosts exists, and some might have seen ghosts (just like someone actually sees a hallucination). All this doesn’t mean ghosts exist, or else the actual concept of non-existence doesn’t exist - which makes the fallacy evident: if we are to consider that all concepts actually exist (further than just an idea), non-existence has to exist.