In what cases do you see Erlang's tooking as better than Elixir? I have generally found Elixir's tooling more approachable, but I haven't yet had to do anything difficult.
That being said, Mix does have its advantages in other ways (easier to extend for one-off scripts, for example, since it won't actually need a whole plugin; the ability for mixed-language projects with Elixir and Erlang), and they're clearly the reason (with Hex) why we have good package management today.
Again, I'm not ready to say one language has better tooling than the other; it's just that I feel it's a far cry from the situation from a few years ago where Elixir was just miles ahead -- the gap has closed since then.