That’s right. If you call a function twice, Nix returns two fresh “thunks”. As a person, you might deduce that the thunks should be the same, but Nix doesn’t know that and re-evaluates them all over again.
There is one memorization happening, on the import boundary. So if two parts of the code import <nixpkgs/lib> for example, that will be the same reference. But for nixpkgs, import <nixpkgs> returns a function. That gets memorized but it’s not super interesting for us.