Black belt would use too complicated technics and overthink.
This guy just hits hard, even if you use a fancy block, he will hurt you. Just hit threw it.
1. He's not fighting black belts in this video, so there's no proof to your claim.
2. Your profile lists boxing as your martial art, so it's no wonder you think someone as limited as this guy would defeat black belts. Your opinion is going to be skewed. (and before you accuse me of doing the same thing with my art, I've done many martial arts and wrestling and amateur boxing. Pure boxers have never fared well in the UFC, think about that, especially in the 80s and 90s when there were know weight classes and fewer rules).
3. This guy is strong, but a one trick pony. He is also fat and slow and is obviously winded by the end. A martial artist would play around with him to tire him out, then kick out his knees.
4. A martial artist, a good one, wouldn't just try to block this guys haymakers. A black belt should be able to avoid them easily, then move in for the attack to his knees that are already week from carrying his fat butt. Look how he walks, he is not limber and walks stiff legged because of his weight and his low state of fitness.