It was just a bad fit for the problem at hand. It was picked mainly because there were a bunch of Elixir backend devs in the organization that were out of work and every problem looks like a web service (or any other system architecture you're familiar with) if you want it badly enough. There was a bunch of hardware interaction that ended up getting externalized into a separate C++ component that exposed a gRPC interface towards the Elixir component both because of Conway's Law and because nobody wanted to (or knew how to) write NIFs and deal with blocking operations on the Erlang side.

A two-digit number of these devices was meant to form a cluster of sorts and using Erlang clustering sounds like a nice solution for that until you realize that the base load of the full mesh that this implies is high enough to use a meaningful chunk of the device's resources.

I would guess they compiled locally.