It's written in that book that are his character stories that he's actually never spoken about it nor expressed any desire to join the Genius Society out loud, it's just widely speculated that he once harboured a desire to. This is in his CS3. I think it is true that he did once, since Aventurine said he thought he'd given up on that already, but has since stopped pursuing that goal, since he knows, now, as he says himself, that with or without Nous, his contributions to humanity are still invaluable.