The application doesn't care if it's "just a transport" or "part of an elaborate spec". Functionally there is no difference between an HTTP transaction and a message queue transaction, for a specific kind of message queue. You implement them literally the same way in an application; open socket, connect, send, receive, optional loop, close.
The point is that a message queue is just a concept. It can exist in many forms for many different use cases. You evaluate the type of queue and implement something that provides for its requirements. HTTP does that inherently for certain message queue types, and for others it requires more code.
Another example is the perennial meme of "PostgreSQL queue". A database isn't a message queue, it's a database! Yet people throw some crap into the DB and call it a queue. Because the application doesn't care how you implement the logical concept. HTTP, RDBMS, MQTT broker, whatever.
