It seems to be a mix of languages and frameworks. For example, it doesn't list Python, but it does list Django. No Javascript, but jQuery and Node.js (Angular programmers are out of luck).
Edit: If you expand the list of supported languages, it shows Python too.
On Runnable you can create basic client-side JavaScript files to be run in the browser (without jQuery) and do window and DOM manipulations.
Node.js isn't a framework for the browser (like jQuery), but a server application platform (with a bunch of objects and functions).
"Pure js" doesn't exist as an environment. It runs within a parent environment (e.g. a browser or a server) and manipulates the global objects in that environment.
JavaScript is just a language specification, and can be implemented in any environment. (For instance, it's implemented in Adobe Illustrator.)
The project template comes with jQuery pre-installed. However, it can easily be removed.
I guess the people behind Runnable assumes that it's more convenient to have to remove jQuery than having to add it for each new JS project, and personally I agree with that assumption.
Hope this helps!