"Looking for Ruby/Elixir developers" -> They just want Elixir developers but put Ruby to have more applicants.
And in general there is a lot of keyword stuffing in job descriptions.
And in general there is a lot of keyword stuffing in job descriptions.
I've even interviewed at some shops that were Rails-based, but since I had a lot of good OO programming experience, designed and built large-scale systems, had skills with other parts of their stack, I had a better overall skill set than some of their existing Ruby devs. And they accounted for this with part of their onboarding process to get me up to speed in their Ruby codebase....
They have at least touched Elixir code but most of your time will be spent maintaining and/or writing Ruby apps.