That is the half-size engine compared to what goes into a 2500/3500 truck. Might as well compare a diesel Rabbit's fuel economy.
Diesel engines are great for the manufacturers that have to meet tougher CAFE standards but still want to sell their most profitable vehicles, pickup trucks. The customers pay for the more expensive engine, the more expensive fuel, the more expensive maintenance, and the manufactures get better numbers for their fleet average fuel consumption. What a deal - for them!