Based on the battery of 55kWh this is about efficiency, not packing more batteries:

> 6.48 kWh/100 km

At a constant 68km/h in perfect conditions (no up/down) and no additional consumers of energy (climate control).

Even on a real route they apparently got some useful stats with 7.51 kWh/100km including charging. Cool.

My car is around 17-20 kWh/100km at a 64kWh battery for a medium cost car in germany. I wish it was even close to that efficiency.

The lower is your state of charge, the more you are willing to reach the destination with your remanining capacity :) The time spend riding slower is usually not so different than when you drive normally, and it reduces the inconvenience of looking for a charger almost at the goal.

seems you compare apples and oranges, I very much doubt your stated consumption is under same conditions you describe for their consumption

ordinary cars can get to 13kWh/100km which ain't that far from 7.5 you mention