Its a real thing, but rather limited. You made an honest mistake and it probably won't happen again. Also thats a mild effect on your watch. My wife works in a hospital and her automatic Cartier Tank Francaise became magnitized twice. When her watch became magnetized it was off by hours. We realized it was because her department was directly across from the MRI department. We did start looking at antimagnetic watches, but once her department moved to a different part of the building she stopped having problems with her watch becoming magnetized.
The real need for an antimagnetic watch is probably limited to a few work enviroments. I think most people buying antimagnetic watches do it as a novelety. Much in the way that someone has pointed out that people buy diving watches watches with helium escape valves...most owners never do prolonged deep see diving where they are living in an environment with a mixture of helium and oxygen, hence the helium escape valve is unnecessary. I imagine most current Milgauss owners work at a desk with a computer and a very limited exposure to magnetism...hell Rolex could barely give the originals away when they were around in the 1960s and 1970s and eventually just made them a special order item. Rolex revived the name and function when they saw how expensive vintage Milgausses had become.
Thanks for that history lesson, much appreciated