Sorry to intrude (and I understand you have been sidetracked of late, Diamond), but I was interested in the explanation behind the conclusion. Or did I just miss it?
I think the structure was built by ants. I decided that it didn't matter if the guy next to the structure poked a stick in the anthill or not to see if ants came out to attack. The building, regardless of whether it is currently occupied or not, is an artifact of ants, having been built by them. That fact removes the structure from the realm of random chance, and also reveals its contingency. Bits of earth held together by an adhesive to form a dwelling are irreducible to the physical motion and chemistry of bits of earth and adhesive. The indepently given pattern match is that the structure is a dwelling.
Cordially,