Why does he force the case that OOP never flats a 3-bet? Is this GTO? Did I miss that part of the explanation?

Also, stack sizes and bet sizes are fixed in this analysis. That said, he does go through the process a few times to demonstrate how the bet size affects each range. My point is to be careful: Trying to memorize the ranges and %-ages out of the context of bet sizing and stack sizing would be a mistake.