Elixir predates set-theoretic types. Simon Marlow took a solid crack at typed Erlang 30+ years ago and couldn’t make it work and preserve what Erlang is. 9 years later Success Typings was published and Dialyzer happened. Not the best, but far better than what any other dynamic language had at that time, and Elixir had that available from the beginning.

So it is possible new theory was actually needed to preserve everything that was judged more valuable than types.