That was a startup that decided to use Elixir and Phoenix when starting from scratch. There were not many people working with Elixir back then and they found me.
Usually my customers are already established companies, small sized, and they find me by word of mouth. Or, I have a lull between projects and I get in touch with companies I worked for in the past or that know me, and I find one that needs help. It's usually either Rails or Django.