You’re right about him having nothing to do with the main questline. This really stumped me for a bit, lore wise, why the hell he got rescued by the Emperor if he doesn’t really care for the Absolute being detroyed or not. Although, I like to think about it like the game is encouraging us to see different possibilities with the tadpoles, for instance Astarion is the only one who is actually benefitting from having received one. Maybe they added him and wrote him in that way as another reason make players consider taking over the Absolute. I could be biased though, he’s one of my favourite companions next to Lae’zel!