Nope, you're incorrect!
(I'm discussing, not trying to be an ass, I'm a newbie here!
)
Unlike a usual slinky wave, the electric and magnetic vibrations of an electromagnetic wave occur in numerous planes. A light wave that is vibrating in more than one plane is referred to as unpolarized light. It is possible to transform unpolarized light into polarized light. Polarized light...
www.physicsclassroom.com
"...However, nonmetallic surfaces such as asphalt roadways, snowfields and water reflect light such that there is a large concentration of vibrations in a plane parallel to the reflecting surface. "
Don't forget the light isn't only polarized at "none", 0 or 90 degrees, it varies between horizontal and vertical. This is why polarizers for photo/video cameras are mounted so they can turn and the CPL for the A129 can turn.
If we're looking at a dashboard, it's not necessarily fully horizontal so the polarized reflection might not be 100% horizontal. Same as the windshield of a car behind/in front of you where reflections prevents you from seeing who's inside the car.
With a dashcam, most of the time the reflecting angle will be close to flat horizontal when we talk about the general view (the road) but the keyword is "most". If you want to lower the dash reflections, you might have to rotate the filter a bit, even if the "default" might work. One thing we don't know is, the "default" might be just random. We don't know if Viofo even pay attention to how it's shipped since it's quite possible it will need to be adjusted to see a difference between "meh" results and "Great", depending of the car and installation.