So is Aboriginal: https://github.com/landley/aboriginal/commit/cfb7977cc32a80b...

The successor project uses musl: https://github.com/landley/toybox/tree/master/mkroot

First, why not. You could also ask why musl was created at all since uclibc predates it by 11 years.

Second, because this project was started during those 11 years, it dates back to 2007 at least: https://github.com/landley/aboriginal/releases/tag/0.3.0

uclibc was spun off by someone else as uclibc-ng with a release just last month. Wikipedia mentions it, but unfortunately squashed the two together haphazardly.

Aboriginal Linux is older than Musl