> (...) leaving effectively a stone cast of a dinosaur foot, rather than a footprint that has somehow been flipped 180 degrees.

Yes, I also share that opinion. In fact, just by looking at the photo from Nature it seems like the cave's floor is comprised of upper strata that have cracked, broked off and fell down. If just so happened that the layer currently exposed has the imprint of some dinosaur footprints.

Perhaps other visitors didn't noticed it because the previous layer broke off recently?