I get why he'd still be cautious of her, even after essentially confirming the suspicions of also being an otherworlder, but either the translation or the original writing is outright failing somewhere with the fact that she started bringing up items from the game & him taking notice, followed by "but how could she know about events that are later in the game?" like bro she was just talking about being familiar enough with the game to know about a busted-but-unusable gag item, of course she'd at least know the basic plot if she's that into the game lol. you just got hard confirmation of your theory & would just wanna be cautious revealing you're in the same boat because you don't know what type of person she is, not because you have any more doubts she'd understand what you were talking about...