No. Express walk-off has nothing to do with Onboard Airline Check-In.
Express walk off just means that you don't place your bags outside your room the last night and will handle them yourselves on debarkation morning.
Onboard Airline Check in you must be flying on a participating airline on a domestic flight after 11:30 am. If you do that, you place your (checked)bags outside your room the last night and the next time you see them will be at your home airport.
There is also something referred to as "Early Flight Departure." If you have flights that leave before DCL's recommended time, you will need to take your flight confirmation to Guest Services at least two days prior to debarkation. On the last night of the cruise, they will leave instructions in your stateroom that explains the Early Flight Departure process (when and where to meet, as well as breakfast options). As noted above, you will need to keep all luggage with you and lug it all off by yourself.
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