Software features are announced at WWDC, which happens early summer and already happened this year. It's only the hardware that gets announced (and the software released) in September.
Some software features are announced at WWDC, mostly ones that affect developers. Consumer-facing services have also been announced during the September event; it’s not just hardware. Apple One, Apple TV+, and Apple Arcade were all announced during the last couple of September events.
It’s August. New iPhones and iOS / macOS are released in September. If they want to introduce E2E encryption for photos and need this in place to do it, then it makes sense to announce this ahead of time and get the backlash out of the way so that they can announce the headline feature during the main event without it being overshadowed.