It seems to be a pretty good mod. There have been mods before to reverse the magnet the same way but many change the wiring to reverse the winding which can cause problems if there's a safety ground and a signal ground on the same wire. Of course a lot of people just buy reverse wound reverse polarity P90s.
Basically boils down to open the pickup, take out the bobbin and flip it over so that the top becomes the bottom and bottom becomes the top (just clarifying incase people "flip" it by rotating it 180 degrees which wouldn't make a difference), rotate the magnet 180 degrees so N becomes S and S becomes N; now it is reverse wound reverse polarity without having to change any wires and any baseplate ground is still on the ground wire, unlike some other mods that reverse polarity by reversing wires which accidentally puts that ground onto the hot wire And it will cancel hum in the centre position but act as a normal P90 in the bridge or neck position.
Is the P90 one of the pickups where the polepieces are directly touching the coil wire or not, and is it the same for all manufacturers. It's a bad idea to remove polepieces completely if they touch the wire because there's a good chance of breaking the wire when you put them back in; but if they're not directly touching, should be a good mod.