And yet he felt the need to bring his entire legion to face down Lorgar and his 50,000 sons. If he didn't think there might be a need to have them there, why not just turn up with his personal retinue, or whatever contingent would normally be on his flag ship? No, he felt that he had to out number the WBs, just in case.
The more I think about it the more it becomes clear that this is a major plot hole left by Abnett. Going on previous work it's blatantly obvious that he had planned for facing off against the WBs. It's a similar fuck up to McNeil's timeline mistake in TOD. Saying that the idea would be so absurd that a master strategist like Girlyman wouldn't ever contemplate formulating a plan for it is just plain stupid.
Jesus, it's like talking to a brick wall. Whether or not the Emperor ordered him to do it, whether or not the idea was absurd and almost treasonous to him, do you really think he'd go in there without a plan for taking on Lorgar and 50,000 SMs? If you choose to believe that, you really haven't any sort of clue about the nature of the Primarch and Legion that you fanboy to death in every thread.
You do realise the emperor ORDERED guilliman to send his WHOLE legion to burn monarchia to demonstrate to lorgar what real astartes look like right? Holy fug mate. Was guilliman preparing for a fight? Who the hell knows