The device rating is accurate, 3140mAH at nominal 4.2V, so it will need at least 15WH total input to charge up completely.
The Amazon link you provided says 4000mAh and 14.8WH on the package ... but the text description says only
2600mAh lithium-ion rechargeable battery. Average charging time of 2 hours. Meanwhile their 5V/1A rating specified in package, times 2 hours typical charge time specified, provides only 10WH, ignoring all losses.
So Duracell (or possibly a counterfeiter) is fudging their numbers with the 4000mAh version, and you can expect same tricks with their 4400mAh version. They are good for just a single charge for small capacity phones.