Yes, it is relevant.
You’re saying, Forsaken immediately faction swapped from Alliance to Horde. Therefore, Calia, who in spite of being very friendly with mainly Alliance-aligned characters, could swap over to Horde easily to become the new leader of the Forsaken, even if it came out of nowhere. Simple, right? Look, it happened in Andorhal, Silverpine and Darkshore! Completely out of nowhere, Alliance became Horde with a snap of a finger. You say that therefore, Calia = Horde is a simple and sensible thing to do. Full stop, nothing further to discuss.
Not even the details of the snap of the finger that made them faction swap. Not worth discussing, the only relevance you saw to it was Alliance -> Horde, and that’s all the use you’ve made of the three scenarios in Andorhal, Silverpine and Darkshore. Instant Horde, just add undeath, all that needs to be spoken of.
It’s not like the Forsaken have a history since Cata of manipulating the fallen dead into becoming last minute shock troops and then giving them the choice of either getting killed again or getting roped into their ranks, which is a completely different context than being safely raised into undeath in a controlled environment by friends who are there to reassure you when you’re back, then suddenly abandoning them because you’d rather join the faction that opposes that of your friends, knowing that if war breaks out again you’ll have to fight said friends, because you’d rather be the leader of your former people.
Said people, especially the pre-Cata portion, has gone through so many trials and tribulations since the plague of Lordaeron that they’re no longer the people of Lordaeron. They’re the Forsaken. Because they have been forsaken. They’ve suffered, they’ve grown distant from who they were, accepted the darker parts of undeath through their work on the New Plague and eventually even stripped down their old human homes in favor of a new Forsaken architecture. Those who came after Cata adopted the new culture as their own because of their shared suffering - suffering that Calia did not experience, because she was not abandoned by her friends nor cast down by being an undead. At least Lilian Voss went through a similar process as the Forsaken. Forsaken by her former friends and colleagues, forsaken by her own father, all the people she knew and loved. Everything she was, everything she believed in got flipped on its head when she was raised into undead.
Not Calia. She’s the same person in life and undeath. She didn’t change. She hasn’t been forsaken.
But some of the Forsaken instantly switched over from Alliance to Horde either because they were forsaken by the Alliance and turned to the Horde for an alliance of convenience (pre-Cata Forsaken) or through manipulation of their frenzied state after being raised from the dead (post-Cata Forsaken), therefore a sudden switch for Calia’s affiliations to be Horde makes sense? I think not.