because windows isn't bloat enough /runs-away-as-this-joke-might-not-be-funny-toall-xD
VM runs the actual thing, and docker is package manager.
Maybe I would call Docker an installation and running method rather than a package manager in this case.
But my read is that the electron application is out of the docker container. It's distributed as an appimage.
So then what is the docker container doing?
The docker container hosts QEMU and the docker files to run Windows. The author probably chose this method because the dockur/windows project basically automates and simplifies the whole process of installing and configuring QEMU, and installing and configuring Windows etc.