I think their point is that Elixir lets you write

    defmodule Multiply do
      def m9(m1), do: m1 * 9
    end

    # elsewhere...
    defmodule Caller do
      def doit() do
        Multiply.m9(2)
        Multiply.m9("hi")
      end
    end

It won't raise an exception or give you a warning while compiling it (tested with 1.18.4). Even adding

    @spec m9(integer()) :: integer()

above its definition doesn't do anything.