I believe the script you are talking about repackages an existing iso. Debootstick builds one from scratch by pulling all the necessary packages from the repository.

For the underlying process of creating this image it uses debootstrap which is the standard Debian way of creating a full system installation (minus the whole bootloader and iso shenanigans). Debootstick allows most options from debootstrap (aka selecting a distro, release, mirror, extra packages, etc).