Just finished it last night:
The way I read it, it didn't seem to me like Guilliman wanted the IW to reinforce the Imperial Palace, but the Fortress of Hera.
Throughout the whole book, I got the feeling that RG believed that the Emperor would ultimately lose, and thus spent his time preparing for this so he could carry on the "cause" of the Imperium.
I personally don't like where this is going.
After years and years of "was the Lion loyal?!" it seems like we're getting ready to start spending more and more time saying "What about Guilliman!" I don't like the idea that something as... "large??" as RG knowing about the Heresy, and choosing to stay in Ultramar to prepare for the ultimate defeat of the Emperor, was put forth in a 60 page short story.
Ultimately with this new look they appear to be giving to the Ultramarines, I feel a retcon, or two, or three for the future.
Are You Trying To Say Jesus Christ Can't Hit a Curve ball?