Athame != Anatheme
:D
Mind you, I think that under Guilliman, the role of Warmaster would have evolved into a sort of Galactic Governor, and his tendency towards the creation of scalable solutions would've meant that he would not have been under the same pressure that Horus was via governance (as is shown in Horus's reaction to the notion of taxing newly compliant worlds).
Mind you, I think he would have cracked down on the conduct of several legions(WE,NL,WS,etc) to the point that they would not have been viable allies of the IOM. However, I reckon that most of the Primarchs would see that his vision was what was best for the Imperium (seeing the 500 worlds of Ultramar as the utopia it is made out to be in KNF).
A heresy would probably still have occurred, possibly one still lead by Horus.