The iPad Pro, a normal tablet, has an M2 without even having a fan, unlike the AVP, so the chip gets certainly not "incredibly" hot. The idea that a metal frame would contribute significantly to heat dissipation is implausible. It's just a frame. Apple uses metal all the time in other products for aesthetic reason where it isn't at all necessary. The Quest 3 doesn't use metal without requiring a large fan. Let alone a metal frame.