Wow nick. You did the math on that one. Good....
I would also agree that the advancing blade(Down swinging blade) is a little faster through the Relative air flow.. very small percent, but it is there. (And only at a higher speed).
But the down blade theory is not just for High speed.
Yes there are different stages through out the variables. Speed/AoA Relative AoA and Prop pitch. Plus the fix differences with different craft. Plow wash out, Tail dragger or level carriage, plane-ing angel of aircraft during different weight loads.
But I still think there are flaws and Contradictions in Mr X's Quote.
I'm thinking. No flap back in a Fixed Prop. Totally rigid.
I am also trying to think about the difference in the AoA of the prop from center and at the tip.
Think of how a change in the change in relative airflow (That +- 21% you mentioned)would be more of an Affect on the tip of a Prop where the AoA is made minimal due to its higher air speed (Wash out). making it even More Prominent.
Nick, thanks for your feed back. Look forward to more discussion if there is more...