That still makes no sense.
To identify your strengths, you also need to identify your weaknesses. Take for example Power Armour. They know that the weak points are the visors and joints because there's no plating there.
That is the very basic of knowing the theory of taking down marines.
When they are in training, punching another astartes in the chest is going to do nothing, because of the fused and reinforced bone of the ribcage - you can't stab them in the heart or lungs. They have reduced reliance on things like the intestinal system - that leaves at a guess from pov either along the armpit or the neck.
Considering that this Primarch is Sun Tzu, Julius Caesar, Duke of Wellington, Napoleon, Alexander, Hannibal and Rommel all rolled into one, I find it singularly disconcerting that even considering the events on Monarchia he didn't envisage the potential for attacking legions.
The Space Wolves considered themselves "Executioners in the Emperor's role", and that the Ultramarines new that time would come for them to give up the way of the warrior, they have to be intelligent enough to realise that time would have come to kill other legionaries. Why there were no theoreticals for that is pretty criminal for the point of view of a commander.
You can state that "you can't compare with a far future military of transhuman supersoldiers", but from pretty much every high tech super advanced military film and scifi ever pretty much consists of such technology reneging on the humans, without that being a metatype thinking, for such highly capable characters (imagine Littlefinger in the Game of Thrones or Varys with their intelligence and cognitive speed in recognizing the change of political landscape) to not think "oh shit, what if putting Horus in charge of the majority of the Imperium's armed forces was a bad thing?" (which Guilliman, among others, thought in the first place), why was there nothing in place?
I can't exactly say that they'll have imagined a massive starship crashing into a planet and then having the systems star turned into a poisonous radioactive light is pretty out there, but repeatedly harping on the point that there was no "theoreticals" from a wider perspective is absolute bullshit, as is the reason for why Thiel was marked for Censure.
I don't have that many other complaints about it, but that massive amount of self evident logic gap is annoying for me.
I agree. I think many in the series try to depict the loyalist loyalist as innocent and undoubtly loyal to the Imperium and Emperor. Horus Rising
showed Horus' concerns about having the legions in line. It just seems weird that at one point we have many legions who are skeptical and jealous of the new Warmaster and then, all of a sudden, they don't know what the fuck is going on.
The Ultramarines not knowing that is pretty absurd. They embarrassed the Word Bearers and played it so coy in that novel that it does stir my guts.
The realistic view of the legions, which goes back from the beginning should really have been that all the legions were skeptical of one another, and even when they chose sides. Thats absolutely realistic, and I believe we are seeing some of that especially in the traitors but even the Lion and Gilliman seem to be falling into that.