Why does it not matter whether you use @behaviour or not?

If you follow the tutorial here:

You can create a Bandit http server that is upgraded upon request for /websocket to a websocket. This is done by passing into the WebSockAdapter the following command:

upgrade(conn, websock, state, opts)

Upgrades the provided Plug.Conn connection request to a WebSock connection using the provided WebSock compliant module as a handler.

But what is strange to me is the “WebSock” compliant module is this:

defmodule EchoServer do
  def init(options) do
    {:ok, options}
  end

  def handle_in({"ping", [opcode: :text]}, state) do
    {:reply, :ok, {:text, "pong"}, state}
  end

  def terminate(:timeout, state) do
    {:ok, state}
  end
end

It works whether or not you have an @behaviour WebSock in it. So what makes it “compliant”? How does it work as a “WebSock behavior” when it doesn’t even need to be specified as one?