If you're playing smaller class football in Iowa, it's just really hard for the ranking services to get a gauge on how good a player like Dejean is, he's just such a superior athlete to everyone on the field it's hard to judge what you're watching.
He certainly looks to have all the attributes, speed, strength, toughness. I would imagine there will be a steep adjustment curve for him being on the field with players who are his equal or better athletically. That's true of all high school players, but will be more marked for him given the level of competition he faced in high school.