Robby restructured the Imperium. It was left in the control of those who rightly should rule it - humanity. Now he comes back and his initial solution is viewed as the problem. Everything that exists now is a direct result of the blueprint he laid down. He knew what humans were like. What I'm trying to say is, he should only be upset at himself. From OP's synopsis it sounds like the typical 40k irony is slowly coming to light. Guilliman and Lorgar are two sides of the same coin. This isn't new in my opinion. Lorgar saw the need in humans to belong, to have purpose, to fill that spiritual void with some kind of common thread that would unite and strengthen the species. Of course, his solution was the Primordial Truth, which, while valid...was more self destructive than anything else. An enlightened species that knew science, knew psykers, knew the Warp, and knew the Gods would have a stronger cultural core to tackle any challenge, whether it was from within the Empire or self, or without. Guilliman, by taking the reigns, putting astartes in leadership positions, and remolding the flawed Imperium he helped create is not so different from a Traitor primarch in my book. Sure, he knew the Emperor's vision, in part. It strikes me as...a mark of hubris to believe even a Primarch could fully comprehend the Emperor's complete vision, knowledge, and understanding of the universe.
Rambling done. Whatever the hell I just wrote..
I don't think Lorgar thought humans had a need to belong or have purpose. He thought that any society should be based on faith, which is anathema to what the Emperor wanted. A more accurate comparison is that Guilliman put everything into place for humanity to rule their own secular empire, and humanity turned around and decided Lorgar was mostly right. So there's definitely an argument to be made that The Emperor messed up with his whole Imperial Truth thing, but it's not fair to blame Guilliman for that. Guilliman setup things right; no religion, aggressive tech exploration, no one person able to challenge the throne; then humans messed it all up by deciding to embrace religion, stagnate tech, and...ok, breaking up Legions is squarely on Guilliman. But the rest was Big E's mistake.
So of course Guilliman wakes up, looks at the mess regular humans have made of things, does an epic facepalm, and tries to figure out how to fix this without going all Horus on everyone.