It also helps that when Gulliman penned his holy tome and attempted to force its addoption he had the full support of Vulkan(he had lost almost all but few chapters anyway) and the Blood Angels(Sanguinius was considered closest to the emporer even by horus) and that his leigon was the size of the rest of the loyal leigons put together at least. and i would assume that his Codex was somewhat like Sun Tzu's Art of war. ex For fast moving targets this. Force the enemy to pllay to your strenghts instead of trying to fight on their terms. The most unexpected course of action is usually the most effective and other such general advice that can be applicable to any situation.