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.
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.