I disagree. I started with Elixir and its OTP resources are really good. Books like Elixir in Action do a great job.

I read Programming Erlang later, but it was just for fun, and I knew most things already at that point.

I've used Elixir since 2015 and in fact learned it first. I still think "Programming Erlang" is a much better book than any other for actually learning Erlang and BEAM/OTP principles. Erlang as a language is simpler, leaving more time and energy for learning the actual important bits about OTP.

Also Elixir abstracts even more of the OTP and does some automagical stuff with it. Erlang is more explicit, which is better for learning, IMO.