How would a geomagnetic reversal work on a flat earth?
There is evidence all over the world that supports pole reversals, just look into mid ocean ridges or lava flows. When the molten rock is cooled magnetic minerals inside of the rock will orient towards the magnetic north pole. Every 100,000 to 1 million years or so the poles switch, meaning the south pole has been the magnetic north pole.
The wiki describes our magnetic field as a radial magnet, which sure explains why we find minerals pointing towards the North Pole, but when the poles are reversed that would mean the minerals would be pointing all over the place, to whichever "edge" it had the most attraction to. We don't find that, the minerals in fact are pointing in one direction towards the South Pole.