For the Sieg fight, on page 7, for one, the explanation sounds like Goutaijutsu, a Maximum Tournament level technique, and Kenichi is seen moving several feet. Even the position at the top of page 6 is way more distance than then worse Sunkei. As far as internal damage, even Joe Frazier could do that, and he's a worse boxer than Ali. And finally, as far as Odin's countering it, that would work for Sunkei, but it's literally impossible to do that for Musunkei, as there's not even enough room between Retsu's fist and, say, Odin's torso or face. It is literally 0 distance
It is massively more impressive. He's imagining his body has thousands of joints, and obviously it's not as simple as "I imagine it, so it's easy" because 1) Even Kaku calls it the perfect technique, 2) it achieves massively better results than the Mach Punch, and 3) even disregarding what we're shown, not only is the power of imagination shown in Baki like well over 10 times to have a physical effect through Baki's Imagination Style, Baki's Imagine Boxing, Yujiro's Imagine Style, and Nomi's Sumo Spirit, but if it was anything less than as described, people like Baki's who's been shown to almost instantly pick up most every technique he's seen (including Mach Punch itself), should be able to use it all with the added durability to tank the downside.
As far as Nirvana, "He performs a pirouette on one foot, with his arms aloft and hands positioned outwards by his sides, using the rotationary force to either depend from attacks, or to enhance the force of his blows." That's literally what Kid Baki does. Can it do stuff outside of this like rotate attacks he isn't expecting, rotate grabs, or rotate multiple attacks? Sure, but that's just a massively better version of Kid Baki's Rolling Kick, which even Retsu's Xiao-Lee is insanely above. Speaking of, as for feats, Retsu no sold sword slashes from Musashi who's thousands of times stronger
Sure, except Katsumi literally fights Retsu to a draw very shorter afterward, as well as beats Dorian within an inch of his life, so unless Katsumi has AD that we haven't learned about, Retsu and Katsumi were comparable in skill and speed (as their profiles say), but Retsu was physically stronger and Katsumi was massively injured from his fight with Hanayama
Edit: also, no, dude literally just shouts loudly, that's not skill. Skill would be turning air into a bullet to blind your opponent to create an opening
