Fineness ratio--yes, that would be a big part of it. Those large luxury sedans tend to be very long, and that must make lower drag coefficients more easily achievable.
Despite what Koenigsegg says, the Regera is a hybrid--three electric motors, 2 driving the rear wheels and 1 on the crankshaft; electric-only propulsion up to 30mph and gas propulsion above that. Where it varies is in the true single-speed, non-variable transmission that directly couples the gas engine to the rear wheels at the final drive's 2.85:1 ratio. But, as
Car & Driver notes, this system is almost identical to what
Honda uses in the Accord hybrid.
Apparently, Lamborghini showed a car this week to owners, but we won't get to find out what it is until next year.