As a Horde player, let's ignore that Sylvanas was colluding with Helja and we have no idea what she actually promised her. Helja who appears to be working with the Legion. She did so against the Varlarjar who more or less on our side and fighting the Legion aswell. Given what is hinted at she might've outright handed her the Aegis amongst other things.
The initial attack on the Forsaken fleet was somewhat questionable as he didn't know what she was up to at that point, his suspicions and distrust of Sylvanas WAS NOT. Sylvanas is the most moraly black leader left. She tried to pull her usual shenaningans but for the first time in a long time failed at it.
This is also a throwback to Northrend. Where the Forsaken waited for the Alliance to start landing in the Howling Fjord, then attacked them from behind wiping out the entire fleet. They then finished off the survivors and plague bombed the incoming reinforcements. Anyone who brought it up back then was shouted down with "We're not friends, we're just fighting the same enemy!" the same was said when the Orcs attacked the Alliance on another occasion ending with the Scourge wiping out both armies. How come when we do it it's legitimate and reasonable but when the Alliance does it to us it's stupid and we should pull together.
Especially given that Sylvanas was pulling some stuff that went against our whole reason for being there, undermining our war efforts and alienating possible allies for no other reason than wanting a bunch of Valkyrs for herself so she wont die any time soon.
Except it was AFTER he already did that that he inducted my goblin Shaman and the other Goblins into the Horde. That he constantly aided and fostered Horde interests and mostly worked with the Alliance begrudingly. By the end of Cataclysm he had retired as warchief but not as a member of the Horde. Living back in Durotar alongside them. When Jaina approached him for help he told her to gtfo, when she came for vengeance he immediatly jumped into the fray to protect Orgrimmar.
Both during MoP and WoD he was active as a Horde member. He was an integral part of the Horde questing in WoD from end to beginning. Claiming he was neutral and basing that on him "never officially rejoining the Horde" is absurd. Especially given that he never truely left it, merely laid down his mantle of warchief and primarily worked for the Earthen Ring during Cataclysm.