An interesting tidbit, by McNeill. However, it only increases the intruige as to which Legion it will cover.
I'm not too sure how this can possibly relate to the sons of Perturabo, or even Guilliman, for that matter. The former, the Iron Warriors, in terms of society and culture are very Hellenic, where as the Ultramarines resemble a more Byanzantine people; in terms of labelling a Historical period to a Legion which shares aspects.
Either way, the promise of a new arc of stories (Hopefully on the Iron Warriors) culminating in the Cleansing of Olympia and Perturabo's succumbing to jealousy and hatred of Dorn should be excellent in the lead up to Terra.