What am I missing here??
A: The efficiency increase in the diesel engine being used to drive the generator.
Many drive systems do not have the diesel engine directly connected to the drive wheels or propeller and most do not use the battery bank. A sailboat would because it has many other uses for a battery bank, including silent drive mode.
And your are missing the fact that it is not a straight conversion of HP to KW when looking at replacing a diesel with an electric drive. The drive in question is using a 4KW system (7.2KW peak, 48v@150a) to replace a 24HP system. Again efficiencies of the electric system and the inefficiencies of the diesel system are not being taking into account.