Like, if you add a little patch to libpng as an overlay over your entire system, then you'll recompile a lot of stuff, but you can also choose to only use that custom libpng with a specific package.
Even in the latter case, it's true that Nix will always recompile the package that depends on libpng, instead of using dynamic linking and only recompiling libpng. This is because Nix has no way of knowing how your libpng patch will affect its reverse dependency, and because link paths are always content-addressed in Nix for reproducibility.
https://nixos.org/nixos/nix-pills/override-design-pattern.ht...
https://nixos.org/nixos/nix-pills/nixpkgs-overriding-package...
Those are the documentation pages I found right now...
It's basically two steps: constructing the patched libpng derivation, and then overriding the libpng derivation in the specific program.
The first step would be something like
let myLibpng = pkgs.libpng.overrideAttrs
(old: { patches = old.patches ++ [~/libpng.patch]; });
let myProgram = pkgs.program.override { libpng = myLibpng; };
If you have an "overlay" set up in your system then you could also do this as part of that overlay.