Good point! Maybe that explains why they chose it only for the bands. People touch the phone the most at the outsides, so this sounds like a great trade-off between allowing the phone to cool itself (via the aluminum back) while not being hot or cold to the touch when holding it.
Well, the predecessor is stainless steel, which I imagine is closer to the Ti number, and in my experience with 12/13 Pro it's always the back that throws off heat, not the frame.
Anyhow, just the frame is this material, inside its Al.
According to these specs, the alloys thermal conductivity is 6.7 W/m-K: https://asm.matweb.com/search/SpecificMaterial.asp?bassnum=m...
Whereas 6063-T6 aluminium has a thermal conductivity of 200 W/m-K: https://asm.matweb.com/search/SpecificMaterial.asp?bassnum=M...
Even if the alloy in the iPhone is a bit more of an exotic blend, won't it still impact the cooling ability of the phone quite significantly?