Is my explanation of a USB battery pack correct?
Actually, almost all those USB battery packs are based on a single lithium-polymer/ion/something cell. Marketing being what it is, most of the time the capacity advertised on the box is actually the cell capacity. Since those battery cells have a ~3.7V output, the battery pack have to convert the 3.7V into 5V with a boost. In your case, assuming the boost is a good one with 90% efficiency, the number you're looking for is, at best:  2200 x 3.7 x 0.9 = 7.3Wh