The latter. But it’s the preview you’re looking at. You’re not actually taking a photo.

A camera app can't set a phone camera to 10,000 fps. So the question is what does it do?

Due to rolling shutter, you take a “progressive” photo. As a result, if the screen flickers during that time, you see this change in light intensity as horizontal bars.

Thicker bars means a lower PWM frequency, hence lower quality light/brightness control.