I believe you've misunderstood his English. If it takes 1 Joule to accelerate the mass from 0-1000km/h, it will take 4 Joules to accelerate the same mass from 0-2000km/h.
Elon is calling that four times as much as because he's considering the total energy required to accelerate from 0 in both numbers. I think you're just accounting for it differently by saying it takes 3 times as much as the original energy input to go from 1000km/h to 2000km/h.
I'm a complete physics novice, I'm open to being schooled on this if I'm completely missing both yours and Elon's concepts here.
I think panic is correct. "from 1000 km/h to 2000 km/h" means from 1000 km/h to 2000 km/h, not 0-2000 kph. Though presumably Elon was thinking of the latter.
Elon is calling that four times as much as because he's considering the total energy required to accelerate from 0 in both numbers. I think you're just accounting for it differently by saying it takes 3 times as much as the original energy input to go from 1000km/h to 2000km/h.
I'm a complete physics novice, I'm open to being schooled on this if I'm completely missing both yours and Elon's concepts here.