You are correct. I guess it's rare because players of that talent usually play as true freshman.I thought redshirts had nothing to do with draft eligibility? You can enter the draft as a redshirt sophomore.
wikipedia
Eligibility
Football players who have been out of high school for at least three years are eligible for the NFL draft. The rules do not state that a player must attend college, but virtually all of the players selected in the NFL draft played college football. A year as a redshirt player in college counts toward eligibility even though the player was not allowed to participate in games during that year. Therefore players who have completed their redshirt sophomore year can enter the NFL draft. A few players are also selected from other football leagues like the Arena Football League or the Canadian Football League.