Earlier quoted context omitted.
I can't really defend their language choice perse, but if you look at the aims of the project - make a terminal that's consistent across platforms and focuses on extendability and interface, what IS your best option? Writing natively for each platform doesn't seem feasible. Using something browser based that has many of those cross platform kinks ironed out doesn't seem like that bad of a choice if that's your top pr…
There's numerous cross-platform alternatives to using JavaScript. Not saying they are inherently better, but there are a hundred different ways to accomplish this.
If your goal is to provide a consistent look and user experience across platforms and be easily extendable, what do you choose? Feel free to name one.