Actually at least one of the vehicles they show in the video does have a turbocharger added to the diesel.
Also, it sounds like they run the engine pretty hard in the demo video. My guess is that you can get away with a less than 100HP diesel if you are (a) willing to find a transmission with a lot of gears, and (b) don't mind running it hard. Those engines tend to develop a lot of torque quickly as they rev up, and most folks driving around don't use anywhere near the full horsepower of their engines. (Except when towing heavy loads...)
I have driven the old 40HP VW vans, and they can accelerate pretty quickly up to highway speeds if you really stand on it. I have also driven 500+ HP cars and they can accelerate way faster than the local police want you to. I've also driven plenty of vintage pickups with 100HP or less.
Lighter, but not necessarily smaller, lower horsepower vehicles are a good way to improve mileage.
A class 8 truck with a GVW of 80K lbs has maybe 500hp but due to the engine’s long stroke they put out massive torque. Divide that hp and weight and a full sized 3/4 ton pickup would be what 60hp? Mileage would be great but I doubt many folks would like 13 gears and a splitter for every day driving.