It's a bit hard to tell, but this isn't open source (well, assuming we all don't consider wasm source :)), right?

I got as far as "powered by cheerpx", which leads me here: https://leaningtech.com/webvm/

And I'm also guessing that you guys just allow free use of the compiled wasm for demos such as this?

Yep! CheerpX's documentation is available here: https://labs.leaningtech.com/cheerpx

Similar project, but open-source: https://github.com/copy/v86

Very nice! Most language playground sites should adopt this approach.

Yup. this is what I use for Yaksha playground (I use v86 though). See -- https://yaksha.pages.dev/