Clearly she's always seen this guy as a guy, he's probably 6'3" with stubble irl, so as soon had he's admitted it and told her his actual name her brain has latched on to it as fitting him much better and so that's what comes to mind first whenever she sees him. Like he says that's what she says when she sees him walk in the door, not what she says in general conversation when she has time to think and censor herself. She likely can't help it.