Its honestly a common misconception...
The computer uses a fuel table that consists of rpm on the X axis and Pressure on the Y axis, with fuel flow amounts in each cell. There is no spark plugs with diesel, so there is no timing table.
So essentially, you can have 1 tune that is calibrated for max economy when crusing around. Once you introduce boost pressure (IE: you step on the pedal and introduce load), the computer is calibrated to pump certain amounts of fuel at whatever cell in the table the engine is operating at during that moment.
The diesel has a turbo. This means the engine can be spun from idle to redline without producing any boost pressure (if there is little to no load). Without boost pressure, this engine is making a LOT less torque and horsepower than these numbers you are seeing. This simply means the engine uses a lot less fuel cruising around than it would require when boost pressure is introduced. Thats what is great about turbo applications.
Even though it may produce 800 some hp and and 1500 some ftlb of torque, it is producing it at max load with probably over 30psi of boost pressure (probably up in the 50's actually). He isnt driving around using 800hp all the time.
This is exactly how the aftermarket computer in my Turbo Buick works, except there is also an ignition timing table to deal with. I can cruise around tuned for maximum economy, but as soon as i get on it and boost pressure builds.... well, it uses lots of fuel and makes lots of power
