There are parts of NixOS that require specific support or you apps simply don’t work, and I can see why a developer would not want NixOS issues causing tickets for them.
1: Though NixOS users have both social and technical reasons for more strongly wanting software to be in nixpkgs than e.g. Ubuntu users have for wanting the software to be distributed via apt.
2. Not wanting people to open tickets/issues is completely orthogonal to being libre software.