The Emperor said that he found the artifact first and that it (Orpheus) freed him from the brain’s influence while he was within it. Then he looked to make allies on the outside, which meant using the power to free the people who were conveniently nearby
I’m sure he tried to pick out the strongest ones that were available, but it’s all presented like he had to be inside in order to wield its power and be free of the brain, and we were allies of convenience.
Not to say you’re wrong, because we don’t even necessarily have to trust his version of events. He was, after all, trying to make sure we would accept him when we discovered he was a mind flayer