It's not as technical as you think and I've experienced it in ALL auto trans cars, even those dating to the '60s with huge 455 cid engines if the hill was steep enough.
I havent heard any of our customers mention it either but it's the torque convertor that keeps enough fluid pressure so that the car doesn't roll back.
On my older Volvo, I swapped torque convertors to a high stall speed convertor (2600 vs 1800 rpms) and it'll roll back in park too at idle. It's not really a problem as your foot's on the brake and unless you're in a place with very steep hills like SanFancisco, the car shouldn't normally roll back much when going from brake to gas pedals.