Gmoccapy tool probe offset problem

I cannot make sense of the Gmoccapy way of calculating a tool offset. From the docs:



In the default macro "change.ngc":

But wouldn't that make the top of my workpiece the zero point of the coordinate system? That is exactly what I experience, so any gcode I try to execute is executed in the free space above the workpiece. Why would the block height be the top of the workpiece, instead of the top of the working surface? Am I thinking about this wrong? Are the docs wrong?