I don't have the training material that you refer to, however I may be able to clarify this for you.
The ONS15540 ESP has a fixed optical backplane which, as you say, provides connectivity between the transponder linecards and OADM's. The optical connectivity across this backplane is independant of the OADM or Mux/Demux module used (ie no different whether 4ch/8ch OADM or 16ch Mux/Demux.
The ONS15540 ESPx does not use an optical backplane, but provides the connectors at the front of the OADM/Mux-Demux and transponder linecards, allowing the user to patch these together, either using direct multi-fibre patch-cords or via a cross-connect patch panel.
Let me know if you still have any questions.