I did find a couple of links that said basically what you have, but they were not referring to battery packs which contained a large number of cells. I believe that the problem is that when one cell fails completely and it could even be the protection circuit... the entire battery pack can typically fails completely, and they are often not rebuildable especially by the end user and not worth a professional’s time to mess with by that point.
As far as buying used batteries that have been “refurbished” this can be a very frustrating game. From experience, they typically are not worth messing with in most applications.
As far as I can tell, all EV batteries are made of many small cells, although the size of the cells has increased as the technology has been evolving, so there are fewer cells per battery pack now.
I used the word refurbished but I think what they’re mostly doing is actually recertifying them. That is, they’re pulling them maybe wrecked cars and putting them through a bunch of tests and if they’ve still got good capacity then they resell them. I don’t know if anyone is attempting to rehab bad batteries.