I guess is depends on what you're working on. There are a ton of things I've needed to understand about Erlang. gen_statem, gen_tcp, gen_udp, gen_event, crypto, ssl, etc.
Yes I agree, depending on the project you might need to dip into some Erlang libraries. But that's a far cry from needing to learn Erlang first.
I remember reading things like "you should learn Erlang before learning Elixir to be really effective" scared me off learning Elixir for a little bit as well.
Also, I find the more familiar you get with Elixir, the easier it becomes to read Erlang code.
I've happily dipped into Erlang libs when I need to. I'm also increasingly capable of reading Erlang code though I only write Elixir. I would not recommend starting with all of Erlang before doing Elixir but it may work better for some folks.
I remember reading things like "you should learn Erlang before learning Elixir to be really effective" scared me off learning Elixir for a little bit as well.
Also, I find the more familiar you get with Elixir, the easier it becomes to read Erlang code.