Almost every system scales simply vertically, vertical scaling is (to a first approximation) throwing faster hardware at the problem. If you're IO bound you get a faster IO bus (this is not necessarily easy to do, but if you do it the system scales to the limit of it). If you're CPU bound you get a faster CPU. And so on.

Horizontal scaling is what's hard for a lot of systems, but simple with Erlang and Elixir since they are, by design, meant to support concurrent and distributed workloads. That's why I was asking if they meant horizontal and not vertical.

Erlang and Elixir are not uniquely easy to scale horizontally, but they are (if you don't fight their nature and produce programs that consist of one giant execution thread) ridiculously easy to scale horizontally by tacking on additional CPU cores or physical CPUs.