The idea is that, in many cases, you can create a layer "by hand" without running actual Linux programs. Layers don't need to be pre-existing, the only requirement is that they can be built programmatically (inside the browser, in this case). The demo actually does that: it "manually" creates a layer from the user-specified entrypoint script, then creates an image from the pre-existing base image's layers and the new entrypoint layer.

In a more real scenario, you can e.g., turn pip wheels into layers without actually using docker's RUN command. All it takes is to massage the data from one archive format into another, programmatically. This unlocks lots of potential (e.g., it becomes embarrassingly parallel to build a container image comprised of pip wheels). Combine that with a good layer caching strategy and a registry that takes advantage of it, and you can have near-instant container builds for arbitrary sets of pip dependencies.