It's not complicated. They moved to titanium to reduce weight, but it made the phone operating hotter, so they figured out aluminium worked better after all with heat dissipation. What's more, this is not like iPhone 16 Pro which featured glass over the entire back. This is about decreasing the amount of rear glass to increase the amount of rear aluminium to aid heat dissipation. This combined with the chamber makes the phone more useable. The rumour for 18 Pro isn't a reversal of this, it's purely a colour change. Did you read the article?