Just saw a very similar product - Charging and USB cable with a 2,800mAh battery, selling for <USD10 in a Hong Kong online store here:
https://www.hktvmall.com/hktv/en/main/ASK-Super-Outlet/s/H0972001/Electronics-&-Leisure/Digital-&-Electronics/Digital-Mobilities/Power-Bank/2-in-1-2800mAh-USB-Cable-L16-TypeC/p/H0972001_S_000032
I think similar products are everywhere and probably come from similar OEMs.
2800 mAh???!!!, This is the first time I see a 2800 mAh 18650 cell, I am not saying it's impossible but I also know that many Chinese manufacturers overrate the capacity, sometimes even more than >50% or much higher.
This might be a solid product but I have my doubts.