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.
The run and shoot, spread, air raid, all have their birth in the same DNA. Once considered a gimmick, you see option routes every weekend. Protections may vary, but it’s the same concept. Play ground football with pads.
Any QB who has played for Leach is pretty much a part of his coaching tree. He lets them call and change the plays and really run the offense, so they get to learn from him as any coach working under him would.