Hi @alexdelprete - I hope you are having a lovely weekend!
I assume you are referring to the explanation I posted here? As you found, there are two related issues. The first issue being the package is “unfree” (hence you set the env variable override to allow the building of unfree packages), and the second is that for unfree packages the NixOS binary cache does not exist and the workaround installs the package from its nixpkgs derivation. We are still planning to make the experience around the use of unfree packages much better but we simply haven’t gotten there yet.
As for the warning warning: input 'flox' has an override for a non-existent input 'nixpkgs-flox' , this is completely unrelated and spurious and should simply be ignored. It is an unrelated message we need to squelch - thank you for bring it to our attention.