You're not going to fit a batter of any useful size, along with enough protection for said batter inside a .76mm thick device today. A .4mm liPo (which I'm not sure I'd want bending 100x a day in my back pocket) has maybe 30Mah of capacity - that's just over 10% of a cr2032 (what AirTags use). 1 month battery life is not what I'd consider useful in such a device.

Just because you can get a chip that fits in there, does't mean you can build a product...