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.

IIRC they didn't want distributed via any package manager (apt, yum &c.); nixpkgs was not singled-out[1]. This was a while ago, so I may have misremembered.

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.

I'm not sure NixOS users are that different from users of other distros, all want a tool that properly manages their systems (rather than random packages being scattered everywhere).

They aren’t really open-source then are they?

1. It's allowed to have wants and desires separate from the obligations imposed by the licensing terms

2. Not wanting people to open tickets/issues is completely orthogonal to being libre software.