My take based on the first post was it worked but only was reading 500 RPM with a drill. While for the tach you have they do mention:
They do not mention the pulse or signal amplitude they used. If you had a function generator or even a good audio oscillator it would be easy to run a few signals in and see what it does. Two wire magnetic tach pick ups are pretty common. Unfortunately they don't all output the same amplitudes. Also, the gap between the gear and the pick up figure into things. You may want to give the seller a call and ask how they test the things?
Ron