Forget custom fabrication; how does one know the real, official fabrication isn't compromised? I mean, the original comment was comparing the openness of POWER8 to the closeness of other processors, using only the ability to flash it openly as evidence, but if one is worried about other processors delivering compromised processors, why isn't one also worried about POWER8 delivering a compromised processor?
I guess I don't understand how that's an argument to support "one can't ever be sure what is running on [the others] internally", unless you manufactured the processor yourself from the open design.
Inside which part? I suppose if you're worried about compromises inside the casing, you need to take off the heat spreader and look for yourself.
Anything on-die falls under the custom fabrications exception.