careful with semantics, they are a night elf sub-race. Not the night elves we play, but a different kind, a sub-race of them.
Lets look in a little closer. Remember in chronicles they are described under the night elf section, not a new separate race category and in the text they are specifically stated as a new race in chronicles
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It doesn't specify elf or night elf. You and I infer that it is a new race of elf and night elf because of what we are shown in-game, through context, cinematics, the dialogue and experience we encounter them in etc. Why would we think they are elves or night elves if chronicles only states they are a new race? Because they are shown as still very much being nocturnal elves of the night elven arcane empire, in the night elven arcane culture, with dark skin tones, night elven themes all over and around, in a night elven city. it is both elven and night elven - and ofc, the very Suramar overview on the website the developer specifically tells you these are a new race that isn't actually a new race, but a very ancient one, the night elves of old.
Here he specifically calls them night elves.
They are changed from the current night elf, but they haven't stopped been night elven at all, in fact they are even moreso. Their arcane addiction struggles parallel the blood elves because these struggles started with the highborne night elves, and is why the blood elves had a smaller measure of it. it was a night elf problem that the alliance night elves suppressed by not using the arcane but was still there in the nightborne night elves and the highborne.
They were once night elves like this
- i.e this kind of night elf but no longer.
They are now this version
- which has a new name to mark the distinction (night based name) called nightborne, still night elven just not that kind of night elf you play on the alliance a new kind, new race, more in the sub-race vein - but definitely it's own separate unique night elven based group. this explanation is best i've seen on it.
They are not meant to be night elves you play, but they are meant to be night elven, same group, just a different race of them, a sub race. Whether it will end up being a sub-race like Dark irons are to dwarves or more like worgen are to humans (i.e. made a full playable race even though technically a sub-race) - really depends on how blizzard choose to go forward with them. Chances are they'll just leave them like all the other sub-race groups. But they might get lucky and become playable.