I think this was them giving themselves a backstory for an option to make Dark Ranger a class for both factions, if they ever wanted to. Not to say that I think they necessarily will. They already gave the Nightborne to the Horde, so if we ended up with NE Dark Rangers even as NPC’s, we would have more kinds of night elves than the Alliance, which… would bother me.
I’m not 100% sure, but I don’t think the dev comment that you’re talking about excludes them from ever going back to the Alliance. I thought he was just saying that at that moment- them leaving with Calia, did not mean they were going back to the Alliance. They were just going off somewhere to cry and talk about their feelings together.
In the same way, I think Blood Prince Dreven’s storyline was a way to give them a similar option for a San’layn. With Vampire Kael’thas looking like a remote possibility in Shadowlands who knows where they are going to go with it.