And I think you're missing the biggest bit, there - each set of 8 cores is its own die. That means that if they have a yield of 95% for that one die, they'll have a yield of 95% for a 64 core monster, too. Compare with Intel, where octupling the die area means their yields would go from 95% to 0.95^8 = 66%. (And if they had 50% yields to start, Intel would end up with 0.4% yields after octupling - AMD would still have 50% total)