I confirm that you cannot supply the DK2 board using CN7. Having done that would have imply to add USB certification as 'Bus powered device' (because there is no battery supply like in a chargeable portable device) which would have added unwanted complexity for a Discovery board.
Note also that CN7 could be used as USB host (not sure it is automatically switching on Starter package).
The STUSB1600 is used to show how to manage the CC lines to being able to advertise as Dual-Role Data for Type-C as well as being compliant with the USB standard requirements. As you pointed, not all of its feature are used, but the DK2 could be used as a start point for a more complex custom device involving sink path.
Regards.
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