The conflation of race and faction here is what is confusing matters. There are three distinct political factions based on their political alignment, one horde, one alliance, and the overwhelming majority who are neutral. I would point out that in terms of gameplay, there is only a single Pandaren race option with the choice of faction being made after the choice of race.
I have to say, the Lightforged Draenei are an odd hill to argue from and a poor comparison with high exiles when the Void Elves are a far more appropriate match. Both consist of individuals who were previously bog standard members of their race transformed by outside powers into something unique, and thus were differentiated along ethnic lines and justified their position as Allied races. A lightforged draenei in other words, is as distinct from an ordinary Draenei as a Void Elf is from a Blood/High Elf. Were they as close to Draenei as High Elf exiles were, they'd simply be bog-standard Draenei who had been on their spaceship for a couple of millenia and they would almost certainly not be an Allied race.
Yet the reason you are able to cite previous instances of developers making mistakes is because they have admitted it and taken steps to rectify the damage. Is it not inappropriate to therefore use their own admissions of wrongdoing on occasion to say that because they were clearly wrong on X, and they admitted it, that they are clearly wrong on Y? That seems a bit of a leap. Particularly as the body of official commentary on this topic covers nearly fourteen years, from the Caydiem blue post in 2005 to the leaked CM response from the High Elf discord where a pro High Elfer was told that while they value people voicing their opinions, that that doesn't automatically mean they have to agree with them. That is the critical difference between previous occasions where they were making mistakes, and this debate, that they haven't said they were wrong and they keep reiterating their position on this matter. Those reiterated positions become the developer commentary regularly cited as the underpinnings of the 'anti High Elf' position, the body of evidence we have which we take to these debates to prove the opinions of the pro High Elf side wrong.
After all, when two groups get into an argument, how do you settle it? You cite evidence to back up your position. The inability of the other side to cite evidence,(once again evidence is something the creators have said, something written in an official work or something done in game) means that the other side dedicates itself to proving the official commentary wrong. Disagreeing with another fan's opinion can be considered one thing, disagreeing with a developer because what they say contradicts your own opinion is another thing, but disagreeing with a fan...then disagreeing with the developer...then insisting not only is the developer wrong but that you are right is pure arrogance (you have not done this of course, others have).
The developers stand behind faction diversity as the reason to deny playable High Elf exiles. As they have not accepted this is wrong, their rationales are valid and perfectly citeable in debate.