Elixir and Erlang both yield BEAM bytecode, which is what actually runs. There's zero difference other than a choice in interface design, which is why they're also completely interoperable.
Phoenix is basically a feature wrapper around Cowboy which is written in Erlang and dates to 2011 when it reimplemented Webmachine which ran on Mochiweb which I am too lazy to dig up the actual start year for beyond "had quite an install base when Rails was in alpha."
Again, potentially useful argument, but you're pointing it in a direction that doesn't make any sense if you actually understand how those work.