Can you tell me how to use the XS version as a duffel bag?
No carrying strap and the sewn-in handles are too short to hold. If I see it correctly, it can only be used as a backpack...
Correct, in the new versions (at least from 2017) the carrying straps are too short to use it as a duffle bag, it can only really be used as a backpack. You can of course also carry the "bag" around laboriously on the shoulder straps, but it's not very practical.
i have carried the xs duffel with me as a piece of hand luggage as well as a bag on city trips.
the harness is suitable for both normal carrying and backpacking.