I think they are trying to clarify that the side racks do not include the ability to mount a top case. It should not require you to use their top rack+adapter plate unless you ALSO want to incorporate their Evo system for a top case. Take a look at the mounting instructions for the side racks and you will see that it does not involve the existing grab rail. You can find a copy at this US supplier site here:
Twisted Throttle.
I have only the ALU top rack on my MTS and can tell you it would not interfere with mounting points of their side racks. It mounts soley to the existing grab rail. So you should be able to go with an alternate mounting solution if you decide to add a Givi top rack in the future. I put up detailed pics of the ALU top rack and Evo quick release Givi plate in a post
here. If you were to install the OEM Ducati top case rack, it includes support tubes which use some of the same mounting points used by the SW-Motech side racks. I believe that is the point of their clarification, you would not be able to add the OEM top rack with these side racks installed.
I had the SW-Motech side cases on my Triumph and found them to be great quality.