I am willing to give you the benefit of the doubt and try to clear things up.
As I understand it, everyone’s concerns are less about the original nix itself going closed source and more over Determinate Systems making their own version, Determinate Nix, using exclusive features and vendor lock-in to make it the de-facto dominant implementation of nix, and taking that closed source.
This is a well-known anticompetive strategy called Embrace, Extend, Extinguish (EEE for short).