When I said PCB I am speaking of the slide.
As was I.
The sensors in the read head read the distance between the gaps in the copper pads on the slide pcb
Yes, they read the change in the capacitance at the edges - The accuracy is wrapped up in the measuring of the change in capacity, straight clean edges of all the parts contribute.
The tyre depth gauges use exactly the same heads as the cheap callipers

They're just re-purposing the heads to find a niche in a flooded market place.
I guess the Wixey devices have the same kind of accuracy (~10um) but have chosen not to display it (it's only wood after all)
[edit] interestingly the early callipers used a 24 bit binary output format that resolved to 1/20480th of an inch (~2um) but the LS bits are too noisy for any kind of use.