"Standard iPhone 15 models will continue to feature USB 2.0 speeds, the same as Lightning".
You mean "same as all current iPhones". My iPad Pro 12.7" (2nd Gen) has lightning, yet still supports USB 3.0 transfer speeds. It's got nothing to do with the physical port spec. I suspect they'll just keep using the same USB controller.
It makes sense from a differentiation standpoint; as well as use cases. A pro model owner using RAW would benefit from faster speeds while most base model users are not likely to notice the difference.