I think you've pretty much said why there is nothing to "fix" upstream here. zsh comp was expecting mkdir. If I'd use a completely unrelated program (e.g. tar)

I think you've pretty much said why there is nothing to "fix" upstream here. zsh comp was expecting mkdir. If I'd use a completely unrelated program (e.g. tar) and told zsh completion to handle it as if it were mkdir, nobody would be surprised if things misbehaves spectacularly.