The comment mentions original iPhone screens. They were 3.5" and we hold phones differently because of the sub 4" displays.

The idea back then was maybe to separate the confirm action from other actions located at the bottom bar.

Human Interface Guidelines changed a lot as Apple changed the iPhone form factor. But with a 3.5", most of the screen is "uniformly" reachable and the bottom is slight less reachable as you usually need to flex the thumb.