None of them belonged to the Grand Alliance though. Arthas was never a ruler among the Alliance of Lordaeron and it had split when he became the Lich King’s pawn. As for Kael’thas, his father had cut ties with the Alliance of Lordaeron after the Second War, which is what kickstarted its fall. He was never a leader in the Alliance, old or new, and if he’s linked with a faction it would be the Horde. He was still alive and hadn’t revealed his treachery yet when the deal between the Blood Elves and the Horde was made, he knew what was going on and didn’t oppose it.
It’s what defines their importance to the overall story since the players encounter them more often than non leaders, whether it’s in game or in the books. The Horde’s losses on that front are clearly higher than the Alliance, there’s no refuting it.
And that’s problematic because Thrall’s Horde in W3 was better. Races of the Horde should be allied because they share common values and goals, not because they are considered (or treated) like unworthy creatures by the Alliance. That’s a serious problem with the Horde and what explains that they had to rebel against their leader twice in the WoW timeline already. The writers do not care about making it a cohesive group, they use that as the weakness of this faction since they do not think or act like one and the story shows that it’s not a positive thing in the writers’ minds. They tried to pin the issues of the Horde on the warchief position instead of who had it and replaced it with a council, but the position of high king of the Alliance works the exact same way the warchief did and that’s not a problem to them. They keep using the lamest writing techniques to make the Horde as bad and dangerous as the W2 one, and when they want to make it good it’s through characters close to the Alliance. This faction still exists because the Alliance allowed it too and it’s too obvious, we often get reminded of that in game. Realistically, an organization such as the Horde would have been dismantled in the real world long ago.
To be fair, Anduin didn’t do a thing when they were torturing Alliance citizens too. I thought this side of him would be explored in Shadowlands but no, Anduin in game can only do morally incorrect things when he is mentally controlled and « Shadows Rising » apparently doesn’t exist in the heads of the lore devs 
But Rexxar isn’t a spy from the Horde equivalent of the SI:7 unless I’m mistaken. Shaw is and the Horde leaders know it. It’s just as ridiculous as Velonara knowing that the Alliance player is using a disguise to help Calia in the new quest line and tells them straight that she doesn’t care. Like, in which way shape or form does that make sense?
I didn’t get to speak to him much, but there was always a lot of thought in his posts and he always made good points even if I didn’t always agree with him.
True, it’s just that the devs of this game tend to not follow what they say themselves when it’s about troops or the amount of civilians left of a people that has been decimated. The fact that enough soldiers of both factions were still alive to fight at the beginning of BFA is ridiculous when we know they almost all have been killed during Legion. Like you said, it’s only before Anduin released Saurfang that Greymane told him that the few soldiers in front of them were the only ones remaining, and that they’d have to enroll farmers next. The Void Elves were only a small group of exiled Blood Elves on a floating rock yet there were enough of us to be « considerable allies to the Alliance war efforts » according to Alleria and so on. It makes no sense because Blizz doesn’t follow the rules they set themselves.
I reckon it was different in the first version of the dialogue, then.