Clever sales dudes put the (inflated) battery rating on a powerbank.
But that powerbank has to convert the ~3.7volt internal battery to 5volt USB.
That will decrease the Ahr rating by ~25%, not counting the losses in the boost converter.
Continuous max current draw of a powerbank is limited by that boost converter, not by the battery.
Powerbanks are designed to charge another device.
Most of them have an automatic shutoff when current draw falls below a certain value (device is charged).
That could be a hassle when you want to power an Arduino.
Leo..