I can kinda see the in-character argument that if the Horde stayed there, even if they kept taking horrendous losses, for long enough for the Alliance to finish off Gul’dan and seal the portal, then the Horde and the whole world (of Warcraft) would have had less losses overall. (And that’s why the Alliance is angry, because they feel they could have finished it there if the Horde stayed.)
Buuuuut, since Legion didn’t showcase that the rest of the world was in danger and limited the “biggest invasion ever” to some never-before-seen-but-actually-contains-active-night-elf-holdings-for-some-reason islands in the middle of the ocean, it really doesn’t feel like the failure at the Broken Shore was catastrophic, and therefore doesn’t make the Horde’s retreat seem that impactful one way or another.
For me, personally, I didn’t even think it could be seen as a betrayal until some Blizzard even later when the spokesperson walked on stage and went “Alliance, you didn’t forget that betrayal at the Broken Shore” and my reaction was “Wait, did I miss something? What’s he talking about?”
Like someone else in this thread mentioned, this situation feels like one where Blizz is talking about the complete story that they planned, rather than the expansion-mechanics-limited story that was actually presented to players.