I assume they're using Ah at the cell level (3.7v nominal), and then maybe rounding up slightly.

(This is the absurdity of using Ah to measure the capacity of consumer-oriented power banks. They usually output 5v over USB A or a variable voltage over PD, but measure current at the cell level. Of course this fact or the precise voltage is rarely stated anywhere.)

It actually is, when you read GGP's link [1]

20000 mAh (3.85V/77Wh)

Which is consistent with what the latest mass produced batteries can do.

[1] https://www.makuake.com/project/haribo_dcglobal/

True and the plastics used aren't typical low weight, the attached cord also weighs (which many power banks don't have). I doubt it even uses a GaN DC-DC converter. There's no way this could be lighter than one designed specially for lightness if the capacity was real.

To be fair, as others have mentioned, claiming internal battery amperage compared to output amperage is a common enough fudge that they're not materially lying.