I think you're confusing which HDR is being used. HDR photography, the multi-expose combinations, was probably iPhone 5S but that still results in an SDR image. That's a completely different thing entirely.

iPhones have not captured HDR images until much, much more recently. No earlier than iPhone 12 at the soonest (when they first could capture HDR video), although they keep fiddling with which format they use for the result. iOS 17 was when they added support for displaying these images in UIKit & Swift, which was only like 2 years ago give or take. WWDC '23 was similarly when they started talking about handling HDR images. And they just recently announced they'll be adopting ISO 21496-1 at WWDC 2024. ISO 21496-1 being the gainmap style approach that Google & Adobe adopted with UltraHDR in 2023.