Well they’re not advertised to be of any specific brand so technically they don’t need to be genuine, but 2,000mAh is an easily achievable number even for generic chinese cells, unless the manufacturer really wants to rip you off I don’t think the cells in those $20 packs are necessarily bad.

Another thing to consider is that the battery pack was designed to use with older cells, and probably has LVP as high as 3.2V. In such case there would be little benefit going for 3,500mAh cells which will provide full capacity only when discharged down to 2.5V.