User blog comment:Supahbadmarine/By Tzeentch! Here We Go!/@comment-4608429-20130211210907/@comment-2253300-20130211213450

No. In this case it simply shielded them from detection via prescience. Basically to the Chaos Gods they didn't draw any more attention than any other mortals. Hard to explain really. Now that I think of it the spell would probably have time limit for plot reason. Lasting until they reached a certain age, and could stand on their own. The point is that the Chaos Gods didn't know they were Primarchs.