You can get a Xiaomi 10400mAh powerbank for $22.22, free shipping from Aliexpress. I'm not saying it's a better deal, but you can already get this raw spec for a lower price. Getting a dumbphone for that extra $3 is quite nice though.

The battery in this phone is only 1,000 mAh, so not that impressive really.

Edit: my mistake, the phone is 10,000 mAh. Impressive indeed!

Using the USB port as a reference, the size of the battery pack is roughly 71mm x 35mm, so it probably contains two 18650 cells side by side.

Panasonic's best 18650s have a capacity of 3350mAh, which sets an upper bound of 6700mAh for the pack:

http://na.industrial.panasonic.com/products/batteries/rechar...

Those "5000mAh" 18650s you find on eBay come from Chinese manufacturers who stick random numbers on cylinders, so the actual capacity is probably off by a factor of 3-5:

http://lygte-info.dk/review/batteries2012/UltraFire%20TR1865...

Well it certainly says that on the battery, but I really doubt has that much power in practice.

Check out: https://qzprod.files.wordpress.com/2015/05/powerbankphone4.j...

It's 10,000 mAh.

That's not what the picture in the article shows. It shows 10000 mAh.