The guy doesn't get hit with a left. The punch that lands is the reverse punch which he runs into. He probably only saw the left arm rising and never knew that the right reverse was even there. The effectiveness of that punch was largely due to the aggressive forward motion of the attack. We can actually see his face stop and his feet continue to go under as if he had ran into a clothesline or a low hanging board.However, at 6:56 he steps to his right side while throwing a diagonal (rising) straight left.
When that much forward pressure comes in, then it just multiplies the impact that is going to occur. Speaking from personal experience, the result would have been the same if he ran into a straightened arm that was held stiff. I tried to find videos of people walking into the end of planks to show the effect, but I wasn't able to find any.