The main issue is the handling of security updates within the Nixpkgs ecosystem, which relies on Nix’s CI system, Hydra, to test and build packages. Due to the extensive number of packages in the Nixpkgs repository, the process can be slow, causing delays in the release of updates. As an example, the updated xz 5.4.6 package took nearly 5 days to become available in the unstable branch!
Fundamentally, there needs to be a change in how security fixes are handled in Hydra. As stated in the article, Nix was lucky to be unaffected, but multiple days to push out a security patch of this severity is concerning, even if there was no reason for concern.
AFAIK it was enabled in anything that used the official source tarball. The exploit binaries were added during the tarball build process.
Nope. There were checks of build environment.
Then why did all distros issue a fix for the package?
Because nobody can be sure there are no other backdoors. And, I guess, they wanted to stop distribution of affected source code.