TP-Link explains it at https://www.tp-link.com/us/support/faq/741/
The charge capacity of a battery bank is typically just the charge that can be held by the cells inside. So to get the power, multiply with 3.6 V for a single cell in series.
It is not the most sensible definition for what is essentially a black box with 5 V input and output, certainly not the definition I would have chosen had I engineered this. But that is what happens when there are no formal standards and device manufacturers just keep copying off each other. Any more sensible definition could not possibly be introduced without creating a formal standard, since those definitions would yield smaller numbers and thus not be competitive.
There is one upside to that definition though - phone batteries use the same definition for their charge capacity (mAh at 3.6 V), so if you know the size of your phone battery and the size of your power bank, you can quickly calculate how often you can recharge your phone from that power bank. Of course, that is after you take into account the inefficiencies of charging that TP-Link outlines in the above article.
