I agree with most of your points, hailene.
I get that authors have thematic considerations for the factions they write about. In the Iron Snakes' case, Dan Abnett was motivated to have them not just behave like Homeric-Greek heroes, but to fight as them as well.
In practice, this was shown through a phalanx-style battle (Priad and the five squads taking on thousands of Orks), and in the Chapter's desire to face their enemies in what they considered worthy combat: infantry action. Unfortunately, as beautifully written as the book was, those parts didn't come off as nicely as they could have.
Greek hoplites, for instance, would have been the first to tell you that it's folly to march out a formation that can be outflanked by an enemy with superior numbers. It's why Leonidas and a few thousand of his best friends (not just three hundred Spartans) decided to hang out within the pass of Thermopylae, as opposed to being in the open. In an ideal world, Priad would have thought the same thing. Ditto with Seydon wanting to take on the myriads of Orks in heroic combat without trying to even thin out their ridiculous numbers from orbit or the air.
I think the people who claim the book was written with a mythic angle might have a point, though. This particular novel's themes might have deliberately
been used in such a way as to trump reality and logic. It can't be coincidence that the next time we saw Priad he was being sent to an Ork held planet to keep them busy by himself
for fifteen years!
Don't get me wrong, I still loved both the novel and the short, but there's a part of me that hopes the Iron Snakes' upcoming Space Marines Battles entry shows that, yes, they can win in their own style... but without that style making you wonder if you're reading a story within a story.