So the thing here is docker provides the tooling to produce reproducible artifacts with graphs of content addressable inputs and outputs. nix provides the tool

So the thing here is docker provides the tooling to produce reproducible artifacts with graphs of content addressable inputs and outputs. nix provides the toolchain of reproducible artifacts... and then uses that toolchain to build a graph of content addressable inputs in order to produce a content addressable output. So yes they are very different, but not in the way you are describing. Using nix, just like using docker, cannot guarantee a reproducible output. Reproducible outputs are dependent on inputs. If your inputs change (and inputs can even be a build timestamp you inject into a binary) then so does your output.