"That could work if there were a viable place in the caves for them to squat. As it is, have been out there personally. The place is full of monsters and, if there is some sort of other civilization or encampment out there, we just haven't found it yet."

Not to mention it would be a terrible place to be holing up.

Rey isn't completely discrediting Firo's theory, of course. She's just putting that fact out there.