As the author of that particular tidbit, I can say this: A proper demagnetization requires the generation of a decaying AC magnetic field -- the idea is to reverse the magnetization of the domains in the media, while simultaneously diminishing their strength. This way, the media's magnetization symmetrically approaches zero. Your instincts are accurate: Your procedure basically magnetizes the floppy in one direction, and so you will most likely be disappointed by the results.

All that said, the easiest thing by far is simply to find 720K PC floppy disks (I'm not yelling at you -- just stating an observation; perhaps things are more difficult where you are, but the 'net is everywhere). They are the same media as 800K Mac floppies (only the formatting differs). Many office supply stores either still have some sitting on their shelves, or will order them for you. There are many online sources (still) as well. Using HD floppies in DD mode involves more fuss and bother (and risk) than it's worth, except in emergencies.