Imagine a more competent backdoor attempt on xz(1)—one that wouldn't have been noticed this quickly. xz is everywhere. They could pull off a "reflections on trusting trust": an xz which selectively modifies a tiny subset of the files it sees, like .tar.xz software tarballs underlying certain build processes. Not source code tarballs (someone might notice)—tarballs distributing pre-compiled binaries.

edit to add: Arch Linux' entire package system used to run on .tar.xz binaries (they switched to Zstd a few years ago [0]).

[0] https://news.ycombinator.com/item?id=19478171 ("Arch Linux propose changing compression method from xz to zstd (archlinux.org)")