I know that banks says they're set in different universes, but the Affront/Issorilians are clearly the juvenile version of the Dweller species described in the Algebraist. (Thank you Culture Fandom wiki for this moment of utter nerdery :)
Been a while since I read both books, but other than a vaguely similar shape, what makes the Affront clearly a juvenile version of the dwellers? I read the wiki you mentioned and there is a story suggesting an early version of contact rescued them from dweller hunts, but no mention of where this idea came from. I'm a huge fan and believe I have read everything Banks has written and do not recall the story.
Just checked out the wiki myself. That reads like fanfiction.
This is probably right and very saddening. Oh well, I guess we can still assume that Against a Dark Background is set in the Culture universe, just very far away.
Or it's a simulated world! After all it has things like the Lazy Gun.