This does not seem to be the case, according to Terran Gregory during the Story and Lore Developer Live Stream, at https://www.youtube.com/watch?v=BhpGvgXyi-U&t=38m43s :
- Terran Gregory: If you'll notice at the warfront, all the NPCs that are Night Elves actually have the darkened eyes. So there's definitely a sense that, in proximity, fighting on behalf of the darkened element of Elune under the darkened moon, that's what happens. And whether or not the player has customized themselves, that of course is player choice, but I think in the story sense that any Night Elf that's fighting on behalf of Elune in that confrontation their eyes would be darkening.
I does not appear Nathanos would have won, as Tyrande broke free of the Val’kyr’s chains herself and Nathanos couldn’t do anything to either of them. Nathanos couldn’t do anything to prevent Tyrande from one shotting that Val’kyr.
Game mechanics. The Heroic Tour of Stromgarde achievement wasn’t completable until after the War Campaign finale, either.
And she was wrong, since, short of a player being a Sylvanas loyalist, we don’t even use the Horde army that was within Orgrimmar against N’zoth, and it turns out it is Wrathion helping the player that defeats N’zoth, not anything Alleria advised.
Most things indicate that the Alliance won the Darkshore Warfront. Nathanos is no longer at Darkshore (having headed back to Zandalar before the Battle of Dazar’alor, and then we see him for Nazjatar before he goes back to Orgrimmar, and he doesn’t show up in the Alliance version of the Darkshore Warfront, either). The Horde’s version of the warfront is set right after the Alliance intro questing since the Horde has to save Belmont, and since that’s always the case and generals don’t change like at Arathi it would seem this is a gameplay mechanic like repeating dungeons rather than battlegrounds (Belmont also doesn’t show up in the Alliance version of the Darkshore Warfront, so it’s not like the Alliance players recapture him every time, either).
And even outside the Darkshore Warfront we have support that the Alliance won:
And then Nathanos says the Alliance is winning on all fronts after Dazar’alor, which would have included Darkshore.