Summary: Lots of improvements in a number of areas can make a big, big difference.
Since ~2008 I know they've increased the energy density of their 18650 cells by 20-30%, which would correspond to a 20-30% increase in range no matter what. After that it starts adding up quick.
I wonder if they might end up restarting roadster production. For a small car manufacturer that could even be fairly logical - produce as many as you can for a relatively short period of time(few years), then shut down production for a few years to let the demand recover and grow.
Perhaps more importantly, increasing the range of a car from 250 miles to ~400 also means that you could put a smaller battery pack in that costs nearly half as much, making it more affordable.
It also helps show the longevity of Battery Electronic Vehicles. Though it's only been two years since they stopped producing it, they're still producing not just maintenance parts, but serious upgrades.