Just to be clear, they're ranking the best DEs who played a majority of their snaps, in 2012, in a 4-3. In 2012, 20 of the 32 teams ran a 4-3. So if that's their criteria, then a maximum of 40 of the players should be their team's intended starters (2 starting DEs in a 4-3). So any player, ranking above 20 is in the bottom half for starters, vice versa, any player scoring 20 or below is in the top half for starters. Like, Vernon, they also considered non-starters as they evaluated the top 50. Though they rated Vernon higher than Odrick, since Odrick didn't make the list. In fairness, Odrick also plays DT.
If interested, here's a breakdown of each team's D. http://bleacherreport.com/articles/1283271-breaking-down-the-defensive-schemes-for-all-32-nfl-teams