1/ Why does Musk keep hinting at his excitement for an electric jet?

In the past he has said that electric flight starts to get interesting once you hit 400wh/kg https://twitter.com/WholeMarsBlog/status/1297765413153992704

2/ Doubling energy density in an EV is beneficial, but the car is on the ground so it's relatively marginal. Removing 150 kg from the Model 3 probably adds ~15 miles of range. Though it furthers its lead relative to the competition.

But for a drone or something airborne...

3/ crudely, the power required to hover is shown in the equation below.
where P is power
L is lift or thrust (i.e. the weight of the drone)
ρ is air density
A is rotor disc area
F.O.M. is the Figure of Merit, roughly equivalent to efficiency.

and L is cubed!

4/ Doubling energy density completely changes the game for any electric powered aerial vehicle.

Any physicists out there please feel free to correct or add on.

5/ And of course the necessary disclaimer that the battery industry is full of "breakthroughs" that never panned out. Though Tesla has repeatedly said it often sees technology before many competitors given it's one of the largest customers of batteries in the world.