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.
You do know the whole Calth thing was set up by horus. Horus odered Guilliman to have his legion at Calth. So, no Guilliman didnt feel the need to have his whole legion there.