Whoever designed these must have slept through thermo (or plans on selling them to people who never took it or also slept through it). If one was to take this device and replace it with the ideal Carnot engine (the mythical highest efficiency heat engine possible) and assuming a 20°F difference in temperature of the ground surface and the boot you'd have an device of about 4% efficiency (real is likely to be less - usually far less). Without knowing the exact construction of the device, you can't know exactly how much energy is available (it depends on things like thickness, area, thermal resistivity of the entire device, etc) but when I read the company's site, I see Seebeck effect, p- n-type semiconductors, thermopile, etc; I just see a marketing gimmick using technical words to convince people, "it can't be fake, it sounds too scientific!" Of course, they missed the obvious marketing line of "employing the same technology that has kept the Voyager spacecraft powered for 20 years" (granted, the Voyager craft uses nuclear decay as a heat source and space as the heat sink so there's considerably larger thermal gradient).
The only way this device has even the remotest chance of working is if the thermopile device has incredibly low thermal resistivity and is very thin (which would end up feeling like you have a hole in your boot - i.e. not compatible with MN winters). Reading further through their website you also see the statement that basically says "12 hours of wear for 1 hour of charge time" leads me to believe you'd be better off buying a little hand crank generator to charge your phone.