It is true. The original post writes about reproducing the minimal iso, which contains probably around 1% of the packages in nixpkgs. The remaining packages are not tested regarding binary reproducibility, or, at least, not in a systematic manner, which means regressions may happen regularly (which is exactly what happened with the .iso, see the previous announcement from 2021:
https://discourse.nixos.org/t/nixos-unstable-s-iso-minimal-x... .)