The Explanation for the events you posted was what I wanted to point out.

To everyone:
Yes, this theory is convoluted and not hinted at in the books before TOD or even the books after that but they do not contradict it.
There is nothing that can't be fixed.
McNeill did this on purpose. Something like that is not written by mistake. It's way too obvious and big.
Other authors confirmed that. Why do some people still think it was all an accident?

Please note I'm not arguing against the opinion that this was probably a bad and unnecessary change.
So far it looks like one to me, too but maybe the Heresy team will suprise us all with a good reason why they changed it.

By the way:
What do you guys think about the possibility that Russ was send after Magnus for his attempt to save Horus?
The Emperor did notice that Magnus projected his spirit across the galaxy with really powerful sorcery.
This could have created a massive disturbance in the warp that pointed straight to Prospero.
All what Horus had to do afterwards was to tell Russ some of the things Magnus did or said during his "healing" on Davin or just tell some lies about it.