They're not using the Zig language. They're using Zig's fantastic packaging of clang with many versions of glibc for easy cross-compilation.
Cross-compilation is the weakest point of Cargo. It depends on a C linker, but can't be bothered to configure a compatible linker. The default linker is for the host OS only, and completely helpless about dependencies, so the out-of the-box experience in Cargo is a wall of errors and disappointment.
Zig build helpfully does all the configuration legwork, and bundles all the required binaries.
