as a child, I was told that my teeth were fairly well protected against cavities on the open surfaces by cursory brushing, by virtue of my sealants- but that flossing was equally if not more important since those inter-tooth pockets were the only places where food could hang out, firmly pressed against the enamel.
There actually hadn't been much research done on it until relatively recently (and probably still not enough), so your dentist was just following the common knowledge of the time.
