How could the Stranger have created Ego? Didn't Ego already have an origin?
The Stranger apparently arranged for other origins for Ego to get out as disinformation.
For those who haven't read AT (the storyline starts well but peters out) The Stranger got certain methods and formulae from the Collector which he used for creating Ego. In return for the Collector's aid, he created a second world called Alter Ego which he gave to the Collector for his collection, and which was imprisoned in an empty universe within a tesseract. The story is about the Stranger arranging for Alter Ego's escape (which Thor unwittingly obtains) in order to reunite the brothers - he knows they will battle each other and he wants to see whether freedom or captivity breeds the strongest will.