Its all circumstantial evidence, no positive evidence (as far as I've seen); and though we wouldn't expect to see a stone at the entrance of the cave saying 937 Hermit Drive, Home of John the Baptist, we have no particular reason to think that this was Johns own actual cave as opposed to the cave of some other hermit who might have looked like John, or a cave that some post-Johannine Baptists used for memorializing John. Man with wild hair and carrying a staff? Must be John the Baptist!It also links to Mark Goodacre's entry which quotes Todd Bolen pointing out that there is a book for sale, and that's why this cave is being connected to the famous biblical figure.
If this was just another Iron Age cistern used by hermits in a later period, no one would care about it. But if it's identified with an important, and little-known, biblical figure as John the Baptist, the potential attention is profound and book sales multiplied.I think it's sad that people's interest in the Bible is being exploited this way.