I don't follow, wouldn't it be faster/cheaper to meet an arbitrary headcount goal using a more-common language or stack?

I think @wkyleg meant that an Elixir dev can be as productive as multiple js devs because of the specificity of the language and the beam virtual machine. Complicated things are trivial in Elixir, dependency management is a breather, and all in all, a really simple and productive environment as the mental model is pretty simple once it clicks.