I already have one of these, think it's safe to use for my passport?
Sure, as long as it fits in your money belt under your pants. If you plan on putting your cell phone and/or passport in this and not otherwise securing it, consider it gone.
I watched the video on their website and understand that it attaches with magnets. I am a runner and traveler and would not trust my passport, credit cards and cash to be secure with magnets. I use an Eagle Creek silk waist wallet for passport, credit cards and cash when traveling (and sometimes I add a safety pin for extra security to the elastic). If I plan on running while I am traveling I take my SPIBelt for my runs to hold my cell phone, ID, and hotel key. I admit I am overly cautious but I've never lost anything.