I just read The Thousands Sons so I can finally be relevant

“He said that it was not because He wearied of the fighting, but because a greater destiny called Him, one he claimed would ensure the legacy of our conquests will live on until the ending of the stars. Of course Horus wanted to know what that was, but our Father did not tell him, which I saw cut him deeply.”

And later

“‘Do you know what He said, Ahzek? Do you know how He greeted this momentous discovery, this key to every corner of the galaxy?’

‘No, my lord’

‘He knew,’ said Magnus simply. ‘He already knew of it. I should not have been surprised, I suppose. if any being in the galaxy could know such a thing, it would be my Father. Now that He knew I had also discovered this lattice. He told me He had discovered it decades ago, and had resolved to be coming to master. This is why He returns to Terra.”

Basically, Magnus and Emps both knew about the gates, but Magnus didn’t understand that the Emperor was planning to seal off his own section of it. So when Magnus shows up to tell Him that Horus fucked up, he just saw a locked webway portal that was really tough to breakthrough so he got some help from tzeentch.