people are quick to assume that if they can't do something, it's impossible.
For this boss, if you don't know you can grab equipped opponent during their wind-up, you're probably not even thinking "there's probably something to do about it, I just don't know what" and just assume it's one of his patterns and just deal with it normally, you're also unlikely to do the right thing by accident, if you've been in combat with any of the guys before, you probably know that your grab key only works when they're down or not seeing you and with the window for it being rather short (for an accident like tha tto happen to happen) and needing to be up close (at least I never managed to grab him out of that from afar) which is something you'll learn to avoid after getting your ass handed to you a couple times
While this makes sense, this bugs me. We all actively acknowledge it's a boss fight off-rip, so why is the first instinct to not treat it like a boss...?
With the game being based on stealth, you really don't have any baseline for how to approach an enduring boss fight, and human first instinct is to punch thing until no move anymore, unfortunately, that's also the boss' strategy, and he's much more proficient at that than you are, and then the issue becomes "game resists, what do ?" and the only logical answer is to call the game bullshit and complain on the internet
Completely understandable, but something that should be mentioned: the third hit can land reliably quite often, in fact. You have to just be aware of certain cues and keep track of how long animations may take.
Optimising and learning where and when you can get mileage of your only good damaging move is really the last thing you're gonna think about when you're frustrated and tilted from not getting anywhere, you probably tried to use it a couple times, never landed it, so you don't even think that it's your fault for using it at the wrong time, you just think "Okay I can't do that" and give up on it
But you're right, whiffing the first two hits and hitting him with the third when he's done guarding is really strong and really helps if you're having trouble with the boss, you can even punish him while he has the bag out and he does the attack that hits in front and in the back, hell you can even punish him for getting the bag out at all, if you do it early enough, he'll drop it and in my experience, he'll often try to pick it back up, which you can punish again the same way until he does something else
Also, I saw someone say they weren't sure the dash was invulnerable or not, and in my testing, it's not, but you have armor (at least against the chop, maybe moves with bigger knockback will knock you out of your dash)