Either Cayde as building toward an appreciatively elaborate prank, or Tony had to start to believe he really wasn't lying when he said he didn't know what the line was for. That sensation of feeling like Tony really should have known could be rationally explained by a frustration with himself, feeling like he was supposed to know everything, but Cayde's struggle to describe this strange gap was relatable enough that Tony hesitated, watching the clock tower like he expected the answer to bloom out of it. Then he was flipping over and swimming a tight circle around Cayde, arguing, "You said you forget stuff when you--...reboot." That was a neat enough explanation, except for the part where Tony had completely missed that he was talking to Cayde-7 now, and his anxious circles frayed with static even as he insisted, "Must have been for something deadly. Clotheslined Jon with it and he obliterated you."