OK, my bad, the photo I saw looked like threads. I guess they were just witness marks from the bolt threads

993 Turbo Rotors are held to the hat with shoulder nuts. Pounded the hell out of mine and never had an issue. Steel hat though. It is a bell shaped hat so that section keeps it true.

If there is concern on the flat aluminum hat from standard plate it dimensional stability. I have machined several different sets of aluminum hats, most half the thickness of the one here. But they have section and a floating rotor. That application assures the hat only drives the rotor and any runout is taken up by the float. Though not as extreme street temps and loads will quickly let you feel any deflection / runout in the pedal.