Souvenir spending is most certainly lower for DVC owners than the average Disney guest, but DVC owners do still spend a decent amount on food and drinks. And food/beverages are still one of the main expenses for every family.
But probably on average (and with Magic Bands Disney certainly knows) less than regular guests. You can put your soda and water in the fridge in a DVC room - and its a full sized fridge in a good portion of the rooms. You can have a breakfast that is bigger than pop tarts or hostess donuts if you are in a room with a stove and a fridge - while Disney's cinnamon rolls are so good, it isn't a huge deal to have cinnamon rolls ready to go as my family gets out of the shower in the morning in my DVC room for a fraction of the cost. As regular guests, we learn over time how big to make the grocery order to have appropriate snacks stashed in our day bags - and from previous discussions on this board, while most of us have a treat or two that is on our must for every stay list - a Dole Whip, a Mickey Bar, A Grand Marnier Orange Slush from the booth in the France pavilion.....we also know what we aren't going to bother with.
I am among the least cost conscious DVCers. But I will admit that while DVC has me spending more at Disney because I bring friends and go more often, we also spend less in water, we spend less in soda, we spend less on breakfast in the morning.