He didn't "brag" about it. He told a story about a meth head who lunged at him and tried to spit on him, so he instinctively used MMA to get her away from him and his dog. Luke is socially awkward and doesn't understand how to tell a story in a humorous way. He's not alone in that regard. Typically, most Americans don't know how to communicate anecdotes well. It takes a very special individual to be entertaining, and Americans have even less experience when it comes to delivering great anecdotes. You're much more likely to be drawn in by a story told by a guy from New Zealand, Scotland, or Ireland--because storytelling is still a trait that is practiced in their culture.
Regardless, it was an event that stood out to him. He tried to relay it in a way that was humorous, but doesn't have the charisma to do so naturally--so it comes off as awkward. That doesn't mean that he was "bragging" or "in the wrong." She assaulted him.
Also, Luke is a former champion in the UFC. He can call out people if he so chooses. If losing to the best fighters on the planet once you pass your prime makes you somehow worthless, then we've completely lost perspective.
Finally, as an added pet peeve, I'd like to mention how we've entered the stage of revisionist history where Bisping is some great hero and Rockhold is a braggadocios villain. In their first fight, Rockhold handled Bisping with ease and was gracious after their fight. He acted like a gentleman. Bisping, when he knocked out Rockhold, called him a fa**ot, made fun of him, and acted like a complete ass. And that's before Bisping went on to duck every contender in the division, eventually losing to GSP who then proceeded to throw the title and the division into the dumpster. And we all know the reason Bisping took the GSP fight, after money, was because he assumed he'd have a dramatic size advantage. Bisping spit on corners, said awful things about people's families, and was hated until he beat Rockhold--which somehow made him a darling in the eyes of everybody who watches MMA. Meanwhile, Rockhold, who is not socially very educated, is a decent guy who just lacks communication skills. Get off it.