The iPhone 18 Pro main camera is f/1.48, which is half a stop wider than the f/1.78 main camera on the 17 Pro. An f/1.48 fixed aperture would probably not be ideal (less depth of field, more revealing of lens aberrations) so there is a real light gathering advantage to the variable aperture.

>Also a phone already starts with very deep depth of field

Fairly deep. You can easily see background blur on a modern iPhone camera if you focus on something close. The very fact that the cameras all have variable focus shows that depth of field isn't as deep as all that. The extra depth of field from a narrower aperture could certainly be useful in some cases, such as landscapes. Here's a question for your favorite LLM:

"If I am shooting an iPhone 17 Pro main camera, what aperture would I theoretically need to get perfect focus from 1.5 meters to infinity, assuming a CoC suitable for a 12MP shot?"

Answer: about f3.2 if you manage to focus at the optimum hyperfocal distance.

>which when stopped down physically reduces the amount of light from the scene that reaches the sensor

As you say, this is only the case if you can't increase the exposure time to compensate. In daylight you will often have plenty of latitude to do so, even without image stabilization. Typical daylight shutter speeds at f/1.78 can be around 1/1000 at base ISO, so there is plenty of room to increase exposure time by 2-3 stops in those conditions.