Maybe I have to re-read, but I think the user is just responsible for keeping the tool within 1/4 inch of the guide line. That's as much correction range as the unit has. The accuracy to the 'plan' is better than quarter inch.
The tape is used because the unit also has to be able to tell how quickly the router is being moved, not just the direction. It has to keep track of the coordinates You could accomplish the same thing with a perfect criss-cross matrix of perpendicular lines, but if there were any error, the output wouldn't be accurate. The tape has a pre-determined pattern on it that the software can measure to get both speed and direction. It's really clever.