They are a model variation, but they aren't exactly the same races either. Nightborne aren't "Night Elves on Horde", they are Nightborne on Horde.
Night Elf on Horde would be Night Elf on Horde. ie- if Broll Bearmantle decided to take a bunch of NE with him so he could be on the same faction as Rehgar and Valeera; that would be Night Elf on Horde.
Other than sharing similar facial features, they have their own model with their own proportions, and Blizzard has gone out of their way to make them culturally distinct from NE.
Blood Elves and Void Elves being regarded as High Elves would make sense though, since they are still directly connected to High Elven culture.