I'm glad we found common ground.

I am not nearly as convinced as you sound that "my stove does not contain X" is a meaningful and accurate thing to say even when X has no extension into the real world at all, but I'm not sure there's anything more to say about that than we've already said.

Consider e.g. "There is not a magical garden gnome living under my floor", "I don't emit telepathic brain waves" or "There is no Superman-like alien on our planet", which to me all are meaningful and accurate, even if they all contain concepts which do not (as far as we know) extend into the real world. Can an atheist not meaningfully say that "I don't have a soul"?

If I adopted your point of view (i.e. talking about magical garden gnomes living or not living under my floor makes no (very little) sense either way since they (probably) cannot exist), then my confidence for or against such a proposition would be equal but very low (no 50% in that case either). Except if, as you say, you're assigning a very high degree of belief into "concept extends into the real world" as soon as you hear someone talk about it.

"This is a property which I know nothing about but of which I am certain that it can apply to reality" is the only scenario in which you could argue for a belief of 0.5. It is not the scenario of the original post.