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.