I don't disagree with you on running games on the BEAM (although slow is a little too far imo), but in this case what they're doing is using Elixir to connect clients with the server. The actual physics simulation and such isn't running on Elixir.

edit: I just read through the article, and it doesn't seem to say one way or the other. 99% sure they aren't running any kind of physics engine on the BEAM, but there isn't anything specific