User blog comment:ImperiexSeed/Supernatural Mysteries/@comment-3481504-20120312215145/@comment-3045936-20120312233126

According to interview with Jared and Jensen. That wasn't a parallel universe or world but an alternate reality that was made. However it was based off the real world hence why sam and dean played their real selves aka Jared and Jensen. All this is on the super-wiki.