Looking at the RTMP specification, in section 5 (RTMP Chunk Stream) it says:
RTMP Chunk Stream includes its own in-band protocol control messages, and also offers a mechanism for the higher-level protocol to embed user control messages.
What is meant by "higher-level protocol"?
It also specifies: "While RTMP Chunk Stream was designed to work with the Real Time Messaging Protocol (Section 6), it can handle any protocol that sends a stream of messages". Aren't the RTMP Chunk Stream and Real Time Messaging Protocol the same?
When RTMP was first released, the Chunk Stream transport worked over sockets or HTTP (though I don't think the HTTP tunneling was ever documented, open source implementations exist). Later Adobe released RTMFP for peer-to-peer communication over UDP. In both these cases, RTMP is a higher level protocol independent of how the RTMP messages are encoded.
For more info, see https://rtmp.veriskope.com/docs/overview/