I don’t think that’s quite correct. For a start, the narrow front track means they can run extremely narrow front tyres, which significantly reduces drag. Tyre wear on the DeltaWing was also extremely low because of the low weight/drag setup; they reckoned they could have done up to 10 stints on a single set. The deltawing shape meant that the floor was the most efficient shape for a venturi, meaning you didn’t get the loss of efficiency you got from a rectangular wheelbase.
Basically, everything which was great about the DeltaWing came specifically from the shape. It certainly wasn’t a gimmick, and they literally wouldn’t have been able to create something with similar efficiency with a traditional wheelbase.