They mean the whole userland. Like, init onwards. It's a great idea because the Linux userland is kind of a hacky mess, and takes active maintenance - not something you really want on an embedded device. This makes it more like a microcontroller, where basically the only process you have to worry about is your own.

My only question is does it work with WiFi? I was under the impression that quite a lot of the Linux WiFi stack was implemented in userland (wpa_supplicant).