Uh, no. The Lightforged are literally draenei with a new body texture and eyeglow. The horns and beards are separate facial options, which is the only thing that is really different. I'll give you that they had to spend time modeling and texturing those, but they can probably justify it with them being relevant in Argus as well as after Argus.
Now Nightborne NPC models? These aren't lacking that much. They have some hairstyles, mostly complete meshes (female models are missing modeled breasts because armor usually covers them anyway, so having those modeled would be a waste of polys - hell, they might even have an earlier version of the Nightborne model that is entirely intact), so they could probably be made into playable models.
Zandalari males could be, too. There is a bare base mesh under it. However, there is no female variant. This would mean that either they have to heavily alter the female troll mesh to fit with the Zandalari, or they have to create them from scratch.
You do also have a point that the fact that the skeletons and animation sets for them already exist is a good thing, as that's the most work, but don't underestimate modeling a new humanoid character and a bunch of hair/face styles.