It's double up. A CPL filter does the following to the picture:
- Removes unwanted reflections from non-metallic objects such as water & glass.
- All the colors are more saturated and look brighter.
- The filter is often used to improve the contrast between the blue sky and white clouds.
- A widely used filter made up of 2 glass slices that can rotate for the desired effect.
That's correct when watching your video. But how much of the light is caused by HDR mode, one can only guess when you have not shot the same without HDR a second time.