Edited for additional thoughts
@ninjadork
He didn't get cut. Assuming it has a sharp edge, a sword moving at that speed should slice through flesh or at least make a gash. There is absolutely no way, from a realistic point of view, that the MC would come out of that unscathed or even moderately wounded. He should have a decent injury blocking a sharp, double-edged sword with just his wrist.
While I'll admit the little black dots may make it seem like he's been cut if you go back a few panels, you'll see the author uses the same black dots to indicate when MC took a hit, but panels after that the MC appears unscathed. That's what I mean.
Granted, this chapter ends without giving us a full view of the MC, but I suspect the author's intention is to show the superiority of karate, from a fictional perspective, and the MC will be unscathed because that's the point of this moment in the story. To show that he can take on a fully armored man wielding a deadly weapon and a shield with his bare hands and come out unscathed.
As to your question.
I've experienced violence. Martial Art, as we've come to understand it today, is a form of learning how to fight with one's body, it is not the end-all-be-all of fighting arts. In reality, Martial Art is simply learning military art forms of fighting which mean guns, swords, hand-to-hand combat, etc. anything that fits into the realm of military combat. Still, what we know of it today came out of people
actually fighting and deciding what techniques best worked. It's not a mysterious mystical thing that is somehow beyond people. I'll grant you that most people have a fictional understanding of the artform just like they usually have a fictional understanding of plenty of things in the athletic realm. I'll also grant you that plenty of physical limitations that people believe in can be broken, but that is abnormal, it is not the normal day to day reality of most people. For most people, even someone well trained like military combatant or special forces operator, they'll tell you that facing off with someone with a weapon with your bare hands likely means you are going to get hurt even if you come out of it with a victory. Hell they'll tell you even if you have a weapon you're likely to come out of the situation hurt, maybe dead. For most people, this is the reality. For a few people, they can achieve the abnormal. But all of us are still mortal and mortality has its limitations.
The problem for me is I see you as taking a fictional understanding from, for sake of argument, stuff in movies like Bruce Lee beating down several sword-wielding opponents and coming out without a scratch.
(Although I enjoy and respect Bruce and Jeet Kune Do) I've been in real fights, been in the military, and while it is possible for a human being to fight barehanded against another human being with a weapon, the odds are likely the unarmed individual is going to incur serious injuries or die depending on the type of weapon unless said unarmed individual has significant training. But said training only gives said unarmed individual enough edge to potentially not die. It's not a guarantee. That's my point.
Whether I know or have seen someone who trained their bodies in the way that the MC has is moot mostly because this is a fictional story and anyone wrestling a bear to the ground bare-handed will fit into that realm of fictional storytelling. You only ever see that type of training in these types of stories. Case and point, if you seriously think you're going to see somebody train themselves like an anime character then go into the wilderness and fight animals unarmed, you're being a little naive here about the reality of human limitations versus animals that are usually severely stronger and faster than us.
People ask if Usain Bolt, for instance, can outrun some of the fastest animals alive like Ostriches and Cheetahs and while analysis shows he could come close, he can't. Usain bolt is an abnormality amongst humanity with his speed due to his training and genetics, but he can't outrun the fastest animals alive.
Real fighting professionals and athletes know their limitations as human beings and thus that's the reason they know to push them. It also allows them to know their disciples' limitations as human beings so they can push them. Anything beyond that is well...abnormal. Again, my entire point on this.
Fair enough, I didn't see where you called him blowing out the back-plate of the armor "Anime Exaggeration" but then again your argument seems to support the idea that this entire thing shouldn't be called abnormal. When in reality, in my opinion, MC should've run to get a weapon to, at the very least, cause some damage on his armored opponent or the author should've researched the weaknesses of armor for the MC to exploit. Thus, as you said, this is obviously intended to be "anime/manga exaggeration" in order to emphasize the superiority of MC's karate and to be an inside joke for the story that MC is obviously already abnormal.