At this point, it's just as productive (perhaps even more as you don't have to glue on a bunch of additional components) and joyful to work with like RoR but massively scalable out of the box thanks to the Erlang VM.
> lead the development of...Our Java, Swift, and React Native-based mobile apps
> - Experience with building application backends in Ruby
> - Experience with React and Redux
> - Interest in machine learning and other next-gen technologies.
That job posting doesn't know what it wants.
I read a post like that and think "why bother when I can get paid the same for doing one of these things well somewhere else".
Offering remote would help.
You can find more experienced Ruby devs -- finding someone with multiple years of Elixir would be very difficult.
My experience with ruby devs is that they are usually employed and not actively searching. Since it isn't the flavor of the week anymore you don't have as many programmers entering the field as ruby devs.
Mentoring is always a solid option too, the language and frameworks are simple. Any CS graduate worth his degree will pick it up in no time.