separatedebuginfo is not supported on darwin. See Darwin doesn't support separateDebugInfo (a fact which is also undocumented) · Issue #203380 · NixOS/nixpkgs · GitHub
Apparently a solution is not very far away bintools-wrapper: Set ZERO_AR_DATE and re-enable LC_UUID on Darwin by zhaofengli · Pull Request #188347 · NixOS/nixpkgs · GitHub but someone motivated and with access to darwin hardware should try to push this through the finish line.