I’m unclear about your definitions. The later pyramids were indeed filled in to be smooth-sided, but that’s just facing. They’re step pyramids at their core.
Sneferu is also credited with building two other pyramids, the Bent Pyramid and the Red Pyramid at Dahshur, which were the first true pyramids to be built as such from the beginning. Though with this innovation the outwardly appearance of Egyptian stepped pyramids came to an end in favor of smooth-sided true pyramids, the pyramids that followed whose cores have been exposed all show some form of stepped pyramid at their center.