the device uses an adhesize magnetic strip that attaches to the print head. The measurer then magnetically attaches to the strip.
ie: no ferrous parts are needed. I've gota roll of magnetic sticky tape in my workshop, useful stuff.

So if the device doesn't need to line up with the printhead - how can you know how far the printhead is from the bed.
I know I'm labouring this point - but I just don't see how it works.