It may work by calculation, sure, but were the Earth's spin the cause of any movement you would be able to recreate the experiment with no motor or push to start on literally any fixture of propper height. that doesn't work. The suspended string would just hang there, unmoving, forever.

Foucault's pendulum shows a latitude specific precession of the pendulum. This means depending on what latitude you are at (which part of the rotating sphere you are on) dictates HOW FAST the pendulum precesses around in a circle. If you change latitudes it precesses at a different rate. If you are on the equator, it precesses not one bit. If you are at the north (or south) pole its precession is maximized and it takes exactly one day to go around. And if you are at the north pole it precesses in the opposite direction as the south pole, exactly as you would expect if you were on a spinning sphere..

This is a simple experiment (although not simple to personally travel to all those places). If you (or any flat earther) had ever studied physics or even just math, there would be no more flat earthers.