Thanks for link to review and discount code.
No that can't be it, as then when battery charged the output should go up.
It makes more sense if you a battery capable of 15W output, and a PSU of 10.5W. It prioritizes the USB sockets then the battery.
The review says it right, you gain simplicity.
(revised, Anker has lowered the quoted weight from 7.6oz to 4.16oz. Typo? As the Slim 5000 is also 4.16oz)
For that you lose Wattage and mah. Powerport2 is 24W. If you simply add the volume and weight of Powerport2 and say the Powercore Slim 5000, you get 10 cubic inch, 7.6 oz, the Fusion is 9.5 cuin and 4.2oz so for 4% smaller size and 45% lighter, you lose 13.5W.