iex runs smoothly because of erlang's console and ExUnit runs in parallel leveraging BEAM, so it's considerable. The foundation was pretty much there from the beginning.
While it's true that Elixir certainly wasn't built from the ground up, and it has benefited heavily from the work of Ericsson on Erlang, Elixir is more than Erlang with different syntax. A lot of stuff in Elixir was rewritten so that people can read the source in Elixir, and not have to know too much about Erlang.