Lightr – Worlds Smallest Smart Charger

The claim is "5V 1500mAh" or 7.5Wh, which does seem entirely plausible in a device that size. (iphone x battery is quoted by wikipedia at 10.35Wh)

Compare to this totally existing product, 2200mAh in a volume of 25mm x 91mm x 25mm https://www.adafruit.com/product/1959

A drawing in the indiegogo page shows 73mm x 27mm, and maybe another image shows 11mm thick but it's hard to say. If so their design has to pack "just" 2x the energy density of a low end product. It might take care in design, but it's not impossible by an order of magnitude like some of these crowdfunding pipe dreams.