See [1]. Essentially the programs which most Linux distributions come with are provided by the GNU project, especially basic infrastructure stuff. Stuff like sed, less, awk, etc.

However these programs have cousins in stuff like BSD or Busybox, which implement similar functionality, but have a mostly, or completely separate codebase, and some differences in what they can do and how you make them do it, with a common user-facing one being differences in the command line flags available, and precisely how they behave.

In this case, Chimera uses a mix of stuff from FreeBSD and elsewhere, and also uses an alternative toolchain for compiling C, called musl, which is not glibc (glibc being a GNU project).

Since Linux distributions that use GNU are almost the universal rule, distros like this describe themsekves as "non-GNU".

https://chimera-linux.org/about/#alternative-userland