Oops, I think I was mistaken. The vanilla nix installation CLI works just fine! I think there might have been some leftover files when I installed it manually from the official website.

curl -fsSL install.determinate.systems/nix | sh -s -- install

Another issue arises from the fact that Determinate Nix manages Nix, but when I try to build my system via a flake, nix-darwin also wants to manage Nix, causing a conflict. Thanks to the @determinate.systems CEO's reply this should be resolved in PR github.com/LnL7/nix-dar...