You mean despite the Codex stating that the Codex is meant to be built on and got around at all times, and not relied on as the be-all end-all? Or did he really expect that should the Primarch still be alive and fight according to his Codex, by using those self-same tactics that got around the Codex when he wasn't present? I really don't think Graham McNeil "gets" 40K.
ADB's Astartes are among the best IMHO - In the Emperor's Gift, he was all "WTFIdonteven..." when he's being flirted with, and even "friendly" behaviour from a comrade like the Mordian Sniper he was more "get off, before I kill you".
Take this for example; an Astartes attempts to shoot someone who's allied, but injured, but slowing down an evac. Another ally steps up and blocks view for the shot. In todays world, no-one would take the shot even after the situation had stopped. The Astartes would likely just pull the blocking ally out the way and then shoot the injured.
To be honest, though, a lot of novels come from the ones who have a strong sense of human loyalty - the Space Wolves from their relatively old acceptance into the development program, the Grey Knights who know that the secret to survival is humanity, Imperial and Crimson Fists, Ultramarines and Salamanders, who have a lot of human interaction.
Consider their alien attitudes coming from the Iron Hands/Sons of Medusa, or the Red Scorpions who have little faith in allies, and rely purely on themselves, as well as having no human presence in their lives whatsoever - they are accepted into the Chapter's development program from birth, after surviving a night naked immediately after birth. The only other chapter like that is the Grey Knights - but they have a lot of interaction alongside other human forces - albeit warped thanks to their ability to request Exterminatus and their Martian allies.
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