Not a video demo, the demo of their product, at https://webcontainer.new/

Why impressive? Looks like every other modern code playground.

The code playground is hosting the WebContainer, it's not the WebContainer. The demo is what's running in the Preview pane.

That took a while for me to figure out, too. I think from the marketing materials Stackblitz uses WebContainers internally as well?

This is one of the reasons I prefer open-source, when the marketing is confusing I can just look at the implementation.