"...any given Tesla battery pack could have lithium cells from more than one supplier... they are all functionally the same. There is really no difference in the product...They do all of the testing on the cells before Tesla receives them. Tesla only takes delivery of cells that meet a certain standard for quality."
From what I've heard (mostly from reading the EVDL) converters, or at least the good ones, try to get batteries from the same production run. This ensures as much conformity between cells as possible.
They may all be 18650 (or whatever) battery size with the same chemistry, but cells from different factories will still be different. After a couple hundred cycles, the weaker and stronger cells will emerge.
As differences arise in each individual cell, it'll get harder and harder for the BMS to balance the pack, which I'm assuming leads to more inefficiency.
I know uneven battery degradation is inevitable, but taking cells from the same production run and manufacturer is a relatively simple step to ensure more reliability and consistency. That in turn keeps battery degradation more in-line.
Maybe Tesla's cell tests (as you say they do) is able to prevent all this. Maybe not.
Considering how paranoid Tesla has been about safety, I'm surprised they aren't employing this relatively simple trick.