When you learn NixOS, you learn NixOS rather than Linux.
That is exactly what I am talking about. You seem to have understood me the most. NixOS could be the unequivocally best distro ever. However, that does not change the fact that a big portion of your knowledge acquisition and experience gained from your time on NixOS, will be for NixOS alone.
I am obviously not putting the two on the same line, but mac shares a lot of terminal syntax and programs with Linux. They still remain vastly different. So, this is exactly what concerns me with the growth that I seek in the Linux ecosystem.