Think of a circle with a fine split in it. At one end there's using a distro. You go around the circle to distroless, and on the other end of the circle, close to using a distro, but not using a distro, is un-distro.
Paraphrased in jest from https://www.youtube.com/watch?v=_OEO79B-CZ4
Seems they use it to highlight it's not just distroless but also kernel-less[1].
[1]: https://www.chainguard.dev/unchained/introducing-wolfi-the-f... (last section)