The Mustang is from before modern safety laws (and feature expectations) and therefore weighed a lot less than your average modern car.
A stock '66 Mustang hardtop had a curb weight below 3000lb, in the lightest configuration close to 2500lb.
Less mass to move will do a lot for efficiency just like aerodynamics will.
Of course, you will also die or be horrifically maimed in an accident in a 1966 Mustang that you might walk away without any serious injuries from in a modern vehicle.
But this conversion is basically just a Tesla model 3 with the shell taken off and a mustang shell installed over. It's mostly a model 3, including the heavy battery and drivetrain. And airbags.
Unless I've missed something here, there are clearly no airbags in the passenger seat or anywhere else in the upper shell (side curtain, etc) from skimming around the video/interior shots.
I guess there's probably still one in the stock Tesla steering wheel/unit.
You also have none of the heavy structure that makes it so you don't die in a rollover or side-impact in a modern car. Look at how skinny that A-pillar is.
Hell, half of this interior is just the raw exposed sheet metal with no insulation/noise-dampening/softer materials either.
A stock '66 Mustang hardtop had a curb weight below 3000lb, in the lightest configuration close to 2500lb.
Less mass to move will do a lot for efficiency just like aerodynamics will.
Of course, you will also die or be horrifically maimed in an accident in a 1966 Mustang that you might walk away without any serious injuries from in a modern vehicle.