[inductive_magic]

To add to the other commenters: when you tweak the sensors sensitivity for light, the aperture will compensate by letting less light in – unless it is fixed. So there should be no noticeable difference in brightness unless you set a fixed aperture.

[foldr]

This is a smartphone camera, so of course the aperture is fixed. Playing around with the app, it does not seem to automatically adjust shutter speed to match changes in ISO. Reducing the ISO does make the image darker – presumably just not as much as OP expected.