The rule also states the foot only has to be in contact with the pitching plate at the beginning of the pitch. If the foot is touching the pitching plate at that start of the pitch, the pitcher is considered to be pushing off from the pitching plate.
It doesnt matter what rule set, even if the pitcher has their heel on top of the pitching plate, as soon as they rock the foot to deliver the heel has lost contact and by your definition the pitcher would be pushing off from a spot other than the pitching plate.
It doesnt matter what rule set, even if the pitcher has their heel on top of the pitching plate, as soon as they rock the foot to deliver the heel has lost contact and by your definition the pitcher would be pushing off from a spot other than the pitching plate.