He should've said he doesn't remember the name anymore and that's it, but he kept yapping. But her reaction is the true fumble, why get mad and just leave the guy like that? She's the one that still remembers and the one that seems to already like him.
I do think they established earlier that he has a habit of getting caught up in the moment, which is a nice bit of continuity.
When she's praising his figurine work he starts getting a bit of a big head about it and basking in the praise, and when he gets into talking bout his craft he starts rambling without meaning to,
plus he's a bit socially unpolished and rambling fits his character in that aspect.
So him just tumbling over into "saying too much" in trying to recall that memory is consistent in what we've established about him in the first place.
Her getting mad at him sells the comedy of the moment --
and sets up him having to go to
her, next chapter, to try and reestablish their connection and progress their shared narrative. Since before she kept coming to him, now that role is reversed at it falls to MC taking up the "active pursuit" role to keep that dynamic from being too one-sided.
Which I think might be part of the reason she storms off like that, from a story writing sense.