Would you call the Night Elves and the Naga the same? No, you wouldn’t, because they are not the same.
Neither are Nightborne and Night elves. They may have started out as the same people but all three races have evovled in different directions, and they are no longer comparable to one another.
Addition: I remember a time when all people asked for in this game was to have a playable naga race. The Nightborne fulfills that fantasy, so don’t tell me they don’t belong in the Horde.