I think that this is a bold statement unless you dig the colony and search for a queen. The same hole collecting ants could collect easily young queens. Nothing prevents the area to be colonised by winged queens origined in the other colony. Not even bats, the surface colony should have a swarm each year.

On the other part, the absence of spiders is much more interesting. Would point to that spiders can't live with a pure diet of ants. Formic acid probably.