The demo's use of Stackblitz is actually really confusing here, because it makes it seem like their IDE is what's being demoed when it's not. WebContainers is

The demo's use of Stackblitz is actually really confusing here, because it makes it seem like their IDE is what's being demoed when it's not. WebContainers is a (proprietary) library for anyone to use to build playgrounds that execute client and server code on the frontend alone. It sounds like it was extracted from Stackblitz's implementation for separate use. WebContainers is not competing in that crowded field (though Stackblitz is) because as far as I know no one else offers a library to build your own. This is partly because it's a pretty niche offering—developers of server-side or full-stack frameworks are the most obvious target users.