This a good design to limit the need to expose more than HTTPS ports.
Connections can be established by HTTPS but then converted into a simple proprietary binary protocol for speedy transfer of data.
No real value in using any standards like websockets since for a grid one is dealing with host to host communication. What ever is fast and simple. Not using websockets makes the protocol more symmetrical.
Once a connection is established, it’s a bidirectional pipe. This symmetry allows one to simplify the design.