Thanks for the scans, Primetime. I didn't know Egron was an established character. You are right, Ewing combined the previous know Ego origins and set it way in the past to give it more scope.
No problem. I think that the original origin is set in the distant past. The reason that we see Thor in the scans is because Ego is causing Thor to mentally relive Ego's origin as if Thor himself is Egros. But those events actually happen well before Thor.
Byrne did a different origin for Ego some time after that contradicts the original origin. So it seems that Ewing chose the original one since it sort of benefits the story.