It looks nice, until the point where he shows the voltages of the cells. 500mV difference I would not really want in my battery pack.
And yeah, you can balance them, just to see them drift away again at each charge and discharge...
On top of this, these cells are LiPo chemistry or similar.
For the price you can get new LiFepo4 cells, thermic recycling for these MG cells would probably be the best option.
In the worst case, these unmatched cells will initiate their own thermic recycling inside your home.