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Yes, I can also confirm that they all belong to Cordyla. These are quite difficult to identify the species, it often requires to see details of cleared and detached terminalia. Very interesting indeed, and new information on this genus as far as I know. Members of the genus are often reared from above ground muchrooms, typically of the genus Boletus. I dont know of any reports from underground fungi, but from your information it might be suggested. Still, February as season would maybe rather suggest some kind of overwintering strategy underground. Many fungus gnats seek to caves for hibernation, but I guess your site was not connected to any larger underground hollows? They could seek down through rather small tunnels dug by for instance earth worms. This is just speculation from my side. Alternatively, they hatch from some underground fungi and crawl to the surface in early springtime. The latter sounds unlikely if the soil was normally packed.