He was extremely revered in junior and even though drouin out scored him he still went #1 and was always projected to be a better pro (although it eas somewhat debated).
One even more shocking thing is he was actually not even the top scorer on Shattuck when he went there, it was a kid named Taylor Cammarata who was drafted but never made the nhl. Mackinnon was also considered a far more promising prospect at the time.
As for his early nhl career, it was 95 percent mental. He showed at 18 he had all the skills necessary to be a superstar. It was a combination of being on a bad team for the first time in his life, and also having to adjust to defenders who could actually contain him.