Board Thread:Warhammer 40k General Discussion/@comment-212.112.154.69-20160521191712/@comment-18016240-20160528061946

The Emperor wasn't quite the psychic powerhouse in the days of the Great Crusade that he is in the "modern" times of the Imperium, as he wasn't being fueled by the Imperial Faith at the time (the power of belief being what largely fuels warp entities and people with a lot of power in the Warp.), he was more like a super primarch than the godlike entity he is now.

We must also take into consideration that up until the rising of the Imperium there weren't many curtails to the Orks post collapse of the Eldar Empire. As such, there were a lot more opportunities for the Orks to grow in power, as when an Ork does a lot of fighting and winning they get bigger and stronger, it's why Nobs and Warbosses are so much bigger and more powerful than the standard Ork. Thus, theoretically speaking it is possible to have an Ork Warboss powerful enough to fight or kill a primarch level opponent, or even potentially the Emperor. I would assume that the Ork who endangered the Emperor's life was in this class.