The ambient light sensor is right next to the camera, I think it's usually to the left. It's a bit hard to test since they're close together. Macs have had ambient light sensors for a long time. For example, take an old iMac and put it to sleep. The power light will pulse with a period of about 2s, and the brightness will depend on the ambient light level.

Edit: Just tested on my MBP. Opened photo booth, covered the camera with my thumb, shone a bright flashlight at the point just left of the camera. The display got brighter but Photo Booth showed no changes in what the camera was seeing.

No, the ambient light sensing is done with a light sensor on the body - look for the tiny hole drilled into the chassis. It doesn't use the camera.