Not that I like what Determinate Systems is doing, but this is not true. Maybe with Nix, but with NixOS and Nixpkgs definitely not.
The last time Eelco left me a comment was in 2017 and he’s de facto inactive nowadays (see this PR for example), so I really doubt he can do “what it wants” with NixOS. Moreoever, even when he was active he didn’t really wield any special power: I can’t find the issue now, but I clearly remember him NACKing a change, being downvoted to oblivion by everyone else in the discussion and then giving up.
EDIT: found the PR I was talking about.