It is not clear what you want to do. Do you want to write the rescue system to a USB stick? Do you want to start a Debian installation from the rescue system?
The instructions that you cited describe how to turn a completed build of the rescue system into something that you can write to the onboard flash memory. UBIFS is meant to be used on top of raw flash devices. You would use a different filesystem with USB flash sticks because wear leveling and defect handling are performed transparently by the stick.
The rescue system is a "Linux" system, but not a Debian system. It does not come with the Debian package management tools and is not comprised of Debian packages. The core of the system is "busybox", which implements most of the typical Unix shell commands in one very compact binary. Jeff has written his Debian installer around "debootstrap" to be compatible with the rescue system. You can boot the rescue system and run Jeff's Debian installer to create new Debian installations on USB sticks.