There’s that, and there’s also the fact that there’s only so many maintainer volunteer manhours. It happens to every distro, it just so hapenned that NixOS was faster this time. Though OpenSuse and NixOS suffer from this a bit less, as they’ve gone out of their way to automate large parts of their update and testing infrastructure(OpenSuse automates everything, I think?).
Part of that is that NixOS had a flake for Plasma 6 throughout the RC phases. So when it hit release, it was mostly just a case of merging it into nixpkgs proper.
Thank @K900@lemmy.0upti.me ;)