It probably doesn't happen that fast. In a mechanical watch, the problem with magnets is that the hairspring attracts itself and such pulls itself together, resulting in the watch beating faster. At least that's what I know and what gets affected first to my knowledge. That pocket watch might have had something like a remontoire spring that got magnetized as well.
Oh. Why isn't a spring drive resistant against magnetism? The problem with mechanical watches is that the hairspring pulls itself together and such beats faster, isn't it? And a spring drive has no hairspring.