I think his conversation with Mat was a clumsily-done attempt to live up to RJ's themes about the origins of legends and so forth, by showing what sort of stories people are making about the adventures Mat has had, of which we know the truth. There have been other instances in the past, which were more smoothly placed, but DomA's explanation of why Guybon's curiosity is plausible and in character is apt. I think Sanderson used his being in a position to hear these sorts of things, and having the opportunity to ask Mat about them to illustrate to the readers the whole origin-of-myths point in a somewhat OoC conversation (Mat was way too forthcoming, given his usual reaction to his legend and his attitude at that time towards nobles in general and Elayne in particular. He'd be far more likely to try changing the subject and dodging the questions than admit or deny as much as he did in as specific a manner as he did).