I don't really subscribe to auteur theory as film is a collabaration. Where directors appear to have a singular style, it is often because they are the tip of a creative iceberg which features the same heads of department across several films. Spielberg is a prime example, as his films generally have the same cinematographer, editor, composer, etc.

All that said, I do believe Spielberg was the driving force in Poltergeist, and that his influence prevailed. :)