Pretty much this.
The Alliance is a bunch of long-time allies who have been trade partners and neighbors since the Curse of Flesh hit them, their Elf-Friends from across the sea who helped them fight against the Orcs, some Space Goats who fought Orcs on another planet, and Wolfmen who were so pissed off the humans didn't genocide all the baby orcs that they built a wall to lock themselves away from the world. (You could also argue that more recently the Worgen are big on Forsaken fighting but the point remains)
Humans/Dwarves/Gnomes have their Titanbond thing going on, historically speaking, but everyone else is "Oh. You fight orcs? We fight orcs. Let's fight orcs, together!"
Where the Horde are refugee conquerors, refugee elves, refugee undead, refugee goblins, refugee tauren... The Horde is all about people banding together to help stay alive, while the Alliance is all banded together to kill them.
And now with the Allied Races we've got Refugee Elves with the Alliance. Refugee Elves who are Refugees -from- the Refugee Elves!
The Nightborn joining the Horde are the least "Refugee" of all of the Horde's races. But even they're recovering from being nearly destroyed by the Legion and seeking alliances with other races to survive.
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Shhhhhhh! You can't use logic like that with Hitei! She's literally excluding -everything- that happened before the Sin'dorei changed their name and eye-color 'cause that's the moment they pop into being as a "New Race", so everything before that doesn't count.
Nevermind that they helped save the world in The Burning Crusade, Wrath of the Lich King, Cataclysm, Mists of Pandaria, Warlords of Draenor, and Legion! She wants discrete concrete examples of when they saved the world all on their lonesome!