Personally I am not willing to use a different IDE/editor for each different technology I use. Thus I'm only considering general purpose editors such as Sublime, Atom, Vi, Emacs.
Worth noting that this uses monaco, the same core editor as VS Code. I'm not quite sure why this couldn't be a set of extensions to VS Code itself, given the number of other extensions... cool result though.
It makes absolutely no sense to build a static website like that with react. If anything, they should be applauded for making the right decisions. Although I agree with tracker1 that having a set of VS Code extensions would have been nice too ;)