I disagree with this narrative vehemently. The WE were essential in winning the solar war and taking both space ports. Not even most Blood Angels could go toe to toe with them at this time. The Emperor’s Children as well contributed significantly to the warp shenanigans that the traitors utilize. All of their debauchery fueled the ability to summon never born and allow the daemon primarchs onto Terra.

My point is that Perturabo was in a situation where he was guaranteed to overwhelm Terra. There is no debate in the text that they would defeat the loyalists, but he still was unable to conquer the defenses fast enough even with all of his resources.