I happen to think that the novel showed us a few things.
First is the possibility that Horus is becoming more paranoid. He did send a captain down first to a planet that was already theirs, after all. Plus the Emperor has had other assassins sent after Horus as shown at the start of the novel. Sure he may have cause to be paranoid, but I think that little bit seemed like it was supposed to show that Horus has started to become paranoid and possibly afraid of his own 'mortality', so to speak, and what his loss would mean to his cause.
Second, that the Emperor's machinations have completely unforeseen ramifications. After all, wasn't it terribly 'convenient' that the assassins just happened to be going to the planet where the item with the Emperor's blood was being held so that they could meet up with Spear and kill him while in the process of prosecuting their assigned mission? It seems like this novel, as well as Thousand Sons, have parts that were specifically designed to show us that the Emperor knows what the hell is going on and what the hell he is doing all the time.
A baby seal walked into a club.