@Luky_o Not trying to find realism in this is the better way to go about HTF, but I think we can just point out that not having chiseled muscles isn't the same as being weak. Man-gi's been depicted as being fairly nimble throughout the series (he runs 30 km every day as part of his ssireum training), so we can assume that despite being fat, the muscles below the fat are actually strong.
What Wan-guk taught him were jabs, which are by definition a whip-like, extremely quick punch. I don't know whether one needs the arm muscles to be hypertrophied to pull them off, but the point of this fight was that Hyeok-soo, who was just a garden variety brawler running on experience and bulk, didn't have the trainin or skill to evade proper jabs. The assumption here is that Man-gi trained enough to be able to jab properly, and that a jab from someone of his size, if it lands, would be extremely destructive. After the first one threw Hyeok-soo off his balance, Man-gi didn't give him time to recover and launched a barrage.
It probably defies credibility anyway… like Hobin never fainting from taking punches from people much bigger than he.