You can learn the syntax of Elixir, Python, Go, whatever very fast, but becoming a master is much less about its syntax - that is usually a trivial part. It is more about underlying standard library, concurrency, collection, execution model, GC, patterns, and caveats.
You might need to learn to read erlang documentation. But I've done quite a bit in Elixir, from standing up websites to writing a VM orchestration engine, and the only things I've needed to read the erlang docs for were:
1) writing an Elixir library that wraps the builtin erlang :ssh module with a more elixirish syntax
2) writing an Elixir library that wraps the builtin erlang :gen_statem module with a more elixirish structure and syntax
3) writing an Elixir library that fixes and wraps the builting :tftp module with a more elixirish syntax
4) figuring out how erlang uses SSL so that I could write a two-way encrypted SSL rpc library.
I did them for fun, but all of these are either in prod or used to make production artifacts, and all of these are of course open source and available, so you can use them and not have to do what I did.
Also FWIW, I have read exactly zero of those books.
I think there's a distinction between understanding and learning a language. In this case, it helps to understand Erlang in order to learn Elixir. Just like you're eventually going to need to be able to read some C if you dive really deep into Ruby.
Difference is that C is probably considered basic knowledge for a working programmer, Erlang isn't. Professional programming eventually leads to being a polygot anyway.