Wow. How? Node.JS needs a "full-blown" OS to compile and run... right? And that's not something you could emulate efficiently using wasm? Right?...?
node.js, as any other native Linux/Windows/Mac application, talks to the OS via a system API. You can emulate that API using WASM, mapping I/O to browser APIs, which is why they mention network is implemented via service workers, for example. File I/O can be mapped to IndexDB... Threads can also be mapped to service workers, I guess... and so on.
emscripten[1] did this a long time ago for C, which is how a bunch of native applications have already been ported to run on the browser.
webcontainers[2] seem to do a similar thing but focused on exposing the browser API to the native apps in a way that integrates well with the JS environment.