Lots of the time that's true. But it's usually coupled with talking so much crap and acting hard to try to intimidate someone. Those are the guys that can't fight.
I know I can fight because I do it all the time. I've done martial arts for years. Yeah, I've had my ass handed to me plenty of times as I've sparred and have been learning from guys who were much more skilled than myself. But, now having years of experience of fighting, sparring, grappling there are a few that can still beat me but they are pro fighters. Here in Utah we have (notably) Court McGee, Demarcus Johnson and Jeremy Horn. All UFC fighters. Those guys would handily embarass me, but for the most part, the other guys around here are as skilled as me or less experienced, etc.
I'm not a guy that wears tapout and struts around hoping people take notice. I could care less.
Anyways, anyone that had trained for a while with mediocre skill and is a decent athlete will destroy any loud mouth who thinks they are tough shiite.
Before I started training, I thought I could handle myself decent, but once I stepped in the ring with guys that knew what they were doing, I realized I didn't know siht.