Look, its not about titles, but actions. During the death star, STH had excellent synergy with the rebellion. The rebellion tag on STH just represents this synergy with the language of the game. There definitely needs to be this complementary game play between STH and the rebellion.
In my opinion, STH became apart of the rebellion once he laid eyes on Leia in the trash compactor and saw her in her wet gown. But he's too gruff to admit to that kind of sentimentality, that he'd join a cause because he was physically drawn to a beautiful princess.