I trained old school obscure TKD (ITF) & after I earned my 5th degree, I switched up and joined a WTF school, earning a 1st degree black belt (kukkiwon). At that time, I was stationed in DC and decided to take up MT at Capital Jiu-Jitsu in Loudoun Co, VA. Folks laughed when I told them I had a TKD background at that school but I was able to handle my own, the entire time, while enjoying the learning of new MT techniques. In fact, I kicked fast as hell and just as hard as the MT guys but had to relearn mechanics. My hands were no where as good but I picked that up fast, as well as combos, angles, clinch game, etc. But again, on day 1, I could already fight and I knew it, regardless of who else thought I would be able to. I think a lot of folks overlook us who grew up in TKD and continued training from childhood well until adulthood. Don't get me wrong, TKD isn't MMA and TKD folks aren't anywhere near conditioned to fight in that type environment. But TKD is a good style and it has its utility. As a cop, I never had a problem fighting people resisting arrest either, even on the ground. I always trained Hapkido with TKD so maybe that' why. In late age, I have picked up Judo and BJJ. I prefer Judo even though it' hard as hell on the body. BJJ is awesome on the ground. Regardless, TKD gave me a warrior's mindset and I've been able to apply it everywhere. So, I don't know what professional fighters have a TKD background but I do know, I have enjoyed a personal amount of success in my own martial arts journey outside of TKD and I am certain it has helped. It has also helped me in my military, local law enforcement and federal careers.
Most people in life have absolutely no training. You take a TKD guy who hits the gym, constantly trains, and gets real world street experience and that TKD guy will use what he knows and adapt accordingly. Just like any other martial artist from any other style will do.