The Planck length is the shortest measurable length, not the shortest possible length. We have no idea how we’d even go about measuring anything smaller, because we don’t understand physics smaller than that. There could be stuff that’s smaller, like even more elementary particles that build everything and determine the rules of the physics and particles that we do understand. Or maybe not. Maybe that’s the smallest anything can be and there isn’t further sub-Planck physics, or maybe it’s just turtles all the way down.