If it isn’t dependent on the hardware, I think you are right.
As it stands right now, the Apple Pencil is dependent on hardware in the iPad Pro and cannot be even used in the lesser iPads. . . much less iPhones or Macs. It requires a multi-touch screen capable of doubling the touch senses of the screen under the tip of the Pencil when the Pencil is activated. . . AND turning off the touch sensitivity to other touches that are NOT the Apple Pencil or in the specific areas of the screen that are controls for the Pencil so the user can actually rest his or her hands or fingers on the screen while drawing.
None of the other Apple devices have this capability or are apparently planned to incorporate that functionality in the future.