He could have easily just chosen to turn a blind eye to Lorgar's behavior instead of humiliating him so tremendously that he went off to the Eye of Terror and brought corruption back to the other legions. It's not like his only choices were 'harshly censure Lorgar or admit to everyone that Godhood was his plan all along'. If the slow pace of the Word Bearer's legion was such an issue, he could have had a private word with Lorgar telling him to speed it the fuck up rather than making such an extravagant demonstration at Monarchia.
Godhood having been his secret plan all along is a fucking stupid Shymaladingdong-esque twist.
My headcanon says it goes back to the Emperors fatal flaw... that he demands total obedience. He knows Lorgar has been doing this for a long long time. He only really cares because it slows down his progress. So he tells Lorgar to cut the shit and speed it up, and Lorgar disobeys. That's the point of Monarchia. Emps doesnt give a shit about the religious shit, but will absolutely not have a Primarch defying his will. The very point of their creation was that they would do exactly what he wanted them to do, unlike the other Perpetuals whose free will and independence fucked up his plans in the past.
Emps is also a micro manager. They write about how when he is looking at the future, he can see the end goal, he just has no idea how to GET there. So he has tendency to try to control all the variables. In a sense, Lorgar may have been a canary in the coal mine. He might have been the first thread of the Emperors Primarch plan to unravel not by getting involved with Chaos, but before that, when he started to diverge from the Emperors will. So the Emperor sees this and tries to crush it utterly, yet it backfires spectacularly. Looking back, most of the mistakes the Emperor made were over-reacting badly to when a Primarch defies him in even the slightest way. He is a really shitty parent.
E to Angron: "Come on, lets get off this backwater and conquer space" Angron "No, my friends" E: "Fuck off kid" Drags him out by his ear
E to Magnus "Stop fucking around with the Warp." Magnus: "But I learned it from watching you Dad!" E: "Do what I say because I said so, oh and here's some public humiliation, Dogboy.... go kick his ass for me"
E to Perturabo: "Shut up Nerd, you'll never be as good as your older brother. Now go clean the shitter"
E about Curze: "Sure the boy tortures animals, its a phase, he'll grow out of it. Besides therapy is expense and I have shit for him to do."
Horus, Fulgrim and Mortarion were kind of entrapped so they don't fall into rebelling. But the other 5 were totally there because the Emperor fucked up handling them.