Not sure what race they can be but from the text alone, it does say anything about predating trolls. It can be a race that predates them but we just don’t know. For all we know that the was a primitive race that evolved into two (via different means). One led to evolve into trolls while another could be the one that the text refers to and eventually became extinct via any number of ways. Some have said that perhaps that same race could have been the same race Bwonsamdi was from.