The torque curve is telling you about the cylinder fill. John's bike is hitting peak cylinder fill at 6500rpm. Below about 4300 it's not filling the cylinder well at all.
You're right, that's a function of the pipe design. Pipes have an enormous effect on cylinder fill because they can pull on the carb during overlap, before the piston even starts pulling on it. That's what his exhaust is doing from 4300 up. Likewise they can push on the carb during overlap which really screws up cylinder fill. That's what his pipe is doing before 4300rpm.
The higher the rpm the engine is turning, the less time available to fill the cylinder. So pipes that pull at low rpm tend to make a higher peak torque number than pipes that pull at high rpm.
Anyway, that's a roundabout explanation of why the torque figure itself isn't higher: it's placed at higher rpm by the pipes, and at higher rpm, there's less time available to fill the cylinder.