I think it's safe to assume he counted "driving layups" and the likes in the IS (inside shots) category.
Personally, if I were to assume that the GE works as intended, I would propose that many users use this tactic incorrectly.
Just like playing "look inside" with 5 shooting guards isn't going to give you a lot of inside shot attempts, people who lack in passing across the lineup may find it difficult to hit cutters and get to the rim, settling for mid-range jumpers instead.
No idea how the statistics were gathered, but I would discard scrimmages and only study teams who use the princeton offense successfully.
My guess is that you need a good to great level of JR/PA/DR across the team (not just a monster PG) to run it effectively.
"Air is beautiful, yet you cannot see it. It's soft, yet you cannot touch it. Air is a little like my brain." - Jean-Claude Van Damme