Erlang lib or System.cmd to other binaries for AWS?

Spawning OS processes is way more expensive than calling local functions (even if the function spawn Erlang processes). Also it is much harder to end with zombie processes (which can be painful in poorly configured Docker containers) or any other issue with running external processes.

At work we built a load generator app in Elixir which uses erlcloud. We are sending 2m SNS messages with no issues (at some point you may notice that the HTTP connection pool becomes the bottleneck, but not the lib itself). Also, calls to erlcloud are wrapped in an Elixir module. Eg.: AWS.SNS.publish(topic, message), this in turn calls :erlcloud.

Is it something you / your team built, right?

Yes. It is as simple as this:

defmodule AWS.SNS do
  def publish(topic, payload) do
    :erlcloud_sns.publish_to_topic(topic, Jason.encode!(payload))
  end
end

Also it helps when unit testing because we can configure a different implementation or use a mock instead of this.