I think there may be different interpretations of what constitutes "sandals". If you're wearing a pair of flat open-toed shoes with no back strap and/or that slot between the big and second toe - that's probably a sandal and probably what they mean. That is a fairly casual shoe even though it's not exactly a flip-flop. I would argue that an open toed dressy shoe with a back strap, not slotted between the toes but happens flat is a dress shoe, not a sandal. That's typically what I wear because I can no longer wear anything with a substantial heel. The only difference between my strappy dress shoes that I used to wear and the ones that I now wear is heel height. And they've always been sold as dress shoes, not sandals.