How is a 10000mAh pouch more energy dense than an 18650?

By not having void air space in between cylindrical cells like you'd get with two 18650s? It's mostly just a matter of packaging.

Pouches can be made to have better volumetric density than cylindrical cells quite easily, which is why laptops aren't generally full of cylinder cells anymore; the challenge is just providing them enough external support/compression and swelling allowance, which can be accomplished through chassis design.

Pouch cells are lighter per mAh due to the lack of metal casing. But more than the casing, the form factor is better for a pocketable device. The cylindrical form factor sticks out, usually wastes some space, and can’t be neatly tucked behind a board or display.