The only reason he could be considered under Leach’s coaching tree is the offense. Yes, it has Briles’ concepts, too, but Leach has a big influence on it.
I haven’t read Leach’s book, but I believe the story goes that OU needed a QB when he was hired as OC. They brought in a couple. OU’s football facility was being renovated, so Leach and Heupel sat on the floor and diagramed plays for a while. Leach said that Heupel was the guy.