There have been snippets of lore to support night elf paladins since the War of the Ancients trilogy, being explained as armored priestesses of Elune.
There are all kinds of race-class combos that can fit into the rigid classes in WoW. Are Tidesage technically shamans or mages? Anduin is still considered a priest despite wearing plate gear. Night elf males can be priests and females can be druids in game, despite older lore stating that kaldorei have fairly rigid gender roles.
There is lore to support night elf paladins, as well as a few NPCs that exist in Legion. That doesn't mean that the Night Warrior customization WILL come with paladins, but it doesn't mean it won't ever happen either.