I was not using the term "common knowledge" the same way Aumann paper was. I was baseing my use of the term on what I found in the lesswrong wiki. I used common knowledge and common priors as essentially the same object in my post. Having the same priors seems to require "all pertinent knowledge" be known by both parties or known in common(this is how I used the term in my post) or a large coincidence where two, pertinent, partial non-overlapping(at least), lead to the same priors.

Imagine I think there are 200 balls in the urn, but Robin Hanson thinks there are 300 balls in the urn. Once Robin tells me his estimate, and I tell him mine, we should converge upon a common opinion. In essence his opinion serves as a "sufficient statistic" for all of his evidence.

May be I do not understand priors correctly. It the provided example it seems like Robin Hanson and and the author have different priors. These two cases seem to parellel what is consider two separate priors in the less wrong wiki:

Suppose you had a barrel containing some number of red and white balls. If you start with the belief that each ball was independently assigned red color (vs. white color) at some fixed probability between 0 and 1, and you start out ignorant of this fixed probability (the parameter could anywhere between 0 and 1), then each red ball you see makes it more likely that the next ball will be red. (By Laplace's Rule of Succession.)

On the other hand, if you start out with the prior belief that the barrel contains exactly 10 red balls and 10 white balls, then each red ball you see makes it less likely that the next ball will be red (because there are fewer red balls remaining).