"No!" he all but snaps it in defiance before he can stop himself. "Well... it's complicated. I guess I wasn't thinking straight at the time so fighting would've a mess." Because it's hard to admit that he's not sure if he could have taken on Xemnas. He did eventually, but that was also with everyone else's help. That said, admitting that Riku was right about anything was a step more than he's willing to take just yet.