Well your entitled to you opinionI'm not so sure for the Theoretical/Practical things, it sounds more like some military manual, which is perfect for Guilliman characterisation as the ultimate logistic/organisational leader.
No doubt that Ultramarines are inspired by the Roman empire, but "inspired" is the key word. Frankly, all Dan Abnett has really done is putting latin names and titles on some characters, and that's all.
When you get a closer look on Maccrage inthe Unremembered empire, it looks more like a modern country of the early twenty century.
And John Grammaticus looks more like a cross between James Bond and Jason Bourne, with a dash of Zakalwe from Consider Phlebas indeed.
That doesn't stop neither to be awesome, tough.