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/