Yeah that is completely true. The theory is that the Emperor made them nascent gods so that they could grow more powerful as time went. Biggest example of this when Corvus beat Daemon Prince Lorgar on a demon world in the eye of terror. It was a horrible beating and as a Daemon Prince Lorgar should have had every advantage but Corvi our grew him. It real interesting that chaos turning the primarchs into Daemon princes actually capped their growth while the primarchs grew.So is it just me or does it feel like Primarchs don't really have a hard cap for how powerful they get? Naturally they'll start having trouble becoming better after a certain point but they're immortal. If they trained for long enough in basically anything it feels like they could become closer to "Emperor Lite" than Emperor/20.