But the wiki also has an embedded script-ish language (letting people create macros/templates for specific purposes and re-use them across multiple articles). That's JavaScript-based, and so is implemented in node.js. And there's a WYSIWYG editor for the wiki pages, which is a couple of MDN-specific plugins we've developed, plus off-the-shelf components (the editor + jQuery, which we also use for a few other things on-site).
So MDN is a Python project with a couple JS utilities attached. And if you set up a local copy, you can pretty easily see that Python is by far the heaviest-used language. But due to the way GitHub counts and reports statistics, it shows up as a JavaScript project.
To pick on an easy example: the copy of jQuery we have in our repository weighs in at just over 9,000 lines. So just having jQuery means you need to write over 10k lines of code in order to get the "real" language of your project recognized.