Your history is a bit off. The Dunmer are very much Dunmer at this time, and the Aldmeri Dominion has existed in one form or another for a very long time. As far as the Pact goes, the Nords helped the Dunmer refugees out of compassion in Skyrim, I see no reason why they couldn't work together to fight a common threat to their existence, even with them still hating each other.
That's a change specific to Skyrim.
I asked because I know the lore but not the dates, hence it seems the first two I got wrong.
The Nords, Dunmer and Argonians hating each other though? No, the argonians used to be enslaved by the Dunmer, they recently [censored] their [censored] up the moment the Dunmer were in trouble and the Nords and Dunmer used to hate each other before Skyrim.