Those are petrified aliens, even in the other set pics you could see "something" that was not an engineer. The petrified look could be, well I would assume it was from an engineers doomsday cure for the black goo, should something go awry. If they have the power to create they surely have the power to destroy.
Still doesn't any one find the similarities between the city and the temples on lv-223 interesting. They look the same aside from rocks in Prometheus there is petrified engineers in Covenant. Same round dome, wall, statues... almost like a time warp.