Thorpe's rankings are based on a combination of skill and the playing time he thinks they will get - it's not about just talent, or who he thinks will have the best career, but what kind of impact the rook will make on his team this year, which is why PT is such a big aspect of it. He's guessing JJ won't get much PT because we have a lot of forwards for him to compete with.
This is what he said in the actual ranking:
James Johnson, Bulls
He showed excellent talent as a passer and had some impressive moments during summer league. But with Luol Deng, John Salmons, Tyrus Thomas and Joakim Noah in front of him at the 3 and 4 spots, how much will he play?
He talks about PT being open for many of the players he's put ahead of JJ. He also updates the rankings regularly through the season, and guys do move all over the place depending on what they do on the court, so there's not much point in getting all pissed at him for thinking JJ won't get a ton of time before training camp has even started.