The whole device is encapsulated for waterproofing so supplying any current at all to it is tricky. It says it's light activated. So likely they've put a tiny

The whole device is encapsulated for waterproofing so supplying any current at all to it is tricky. It says it's light activated. So likely they've put a tiny light sensor in it that the microcontroller polls occasionally during the lowest power mode. That might be a tiny solar panel too. But using that to electrically drive an electronic power switch is tricky if you don't have the budget for a custom ASIC; if you don't have that budget, you'll add a decent amount of weight in a device that tiny. Honestly, I'm not at all surprised that they've chosen to accept a short shelf life instead. It's pretty remarkable that they can squeeze that much functionality into a 11mm long by 3.5mm diameter cylinder.