If I understand correctly, Elixir builds an Erlang syntax tree and then the erlang toolchain takes over. If it runs Erlang, it should be able to run elixir
I think you have it the other way around. Elixir is built on top of Erlang, no? So if it runs Elixir it can run Erlang in the same way React can run JavaScript, but not necessarily the other way around.
No, the GP is correct: Erlang and Elixir are 100% equivalent on the BEAM, because Elixir compiles to Erlang AST, then that is compiled with Erlang compiler to BEAM bytecode. I honestly don't know what you mean by the React example, but it's more like JavaScript and TypeScript, or Java and Kotlin. At runtime, there's no difference between the code compiled directly from Erlang and code compiled from Elixir.