> And finally, we certainly do not ship the build system of glibc with Zig! I manually inspected, audited, and analyzed glibc's build system, and then by hand wrote code in the Zig compiler which hooks into Zig's caching system and performs a minimal build of only these start files, as needed.

It sounds to me like they build glibc using its own build system, but they use Zig to build the statically-linked glue code necessary to dynamically link to a system glibc. This allows cross-compiling to a glibc target without having glibc installed locally. Certainly very impressive.