Probably a missed left over from somewhere in the commit58f385f680. As can be seen in that commit where this line was introduced, "$@" was also just emptied by the last `set` call in line 169. This line is currently valid, but breaks suddenly when somewhere earlier a `set --` instruction is used in the future. Neither in commit58f385f680nor in PR https://github.com/NixOS/nixpkgs/pull/197547 have I found anything stating that this "defect" was intentional.