Actually the greyshields were all loyalist stock. The idea that Cawl used traitor primarch genseeed is just something people wanted to be true, instead of actually being true. 

"Active for over a standard century well into the 42nd Millennium, they were comprised of nine Space Marine Legion-sized armies of Primaris Space Marines, all drawn from the lineage of one of the 9 Primarchs who had led the Loyalist Space Marine Legions during the Horus Heresy."

That is a quote from the wiki page and they do repeat that in the dark Imperium books. 

it seems that, at least for now all known primaris marines are taken from loyalist stock only. 

[@Apollosarcher]