It looks like this is actually just the editor from Visual Studio Online unless I've missed something.
It's great, but if it were really Visual Studio Code that would be awesome and I'd be pleasantly surprised.
(The difference being that if it's just the editor it misses out on all the compiler/analysis integrations. If Github were providing Linux containers in the cloud for working on projects - essentially the SSH feature from Visual Studio Code - it would be absolutely brilliant.)
I think what you might want is GitHub Codespaces?: https://github.com/features/codespaces. Was on HN recently too.