If its the free stream, then I don't agree and I don't agree with the parallax demonstration by Mr Williams either. The reason the parallax arguement doesn't work is the free stream presents itself to the wing from a direction which is parallel to the trajectory of the aircraft. So if you close one eye then look from a point half way down one wing you are assuming that the free stream is approaching both wings from one point. Which it clearly isn't.
If you think that the effect is just a result of parallax then let's adjust the experiment.
keep both eyes open and position the paper so that it is side slipping towards your eyes. Now look directly ahead so that the distance between the two points at which your eyes are focused (one on each wing) is the same as the distance between your eyes. The line of sight from each eye is now parallel to the trajectory of the aircraft.
As before the angle of attack will be greater for the leading (dropped) wing. Taking it to extremes if you yaw the paper to give a side slip angle of 90 degrees you will see the bottom of the leading wing and the top of the trailing wing. This means that the leading wing will have a positive angle of attack and the trailing wing will have a negative angle of attack.
My only reason for suggesting that one eye be closed is that some people experience blurring and double vision keeping both eyes open.
In one of your earlier posts you have stated the following:
The lift vector is perpendicular to the wing. So if there is dihedral, the vertical component of the lift vector is greater from the lower wing which tends to roll the aircraft back level.
You appear to be saying that a vertical component of lift is required to roll the aircraft level. Well let’s consider an aircraft with no dihedral/anhedral in a 90 degree bank. If we apply aileron we will increase the lift on one wing and decrease the lift on the other. This difference in lift will exert a rolling moment. But all of this lift will be horizontal, so if your hypothesis is correct the fact that there is no vertical lift means that there will be no rolling moment. Do you really believe that aileron deflection will have no effect in this situation?