That study is exclusively with smaller groups: 3 people. The social dynamics of a 3-person group are very different than a 10-person group as in this study. This probably has much to do with the different results.
Here is a more scientific summary of the research... http://www.wjh.harvard.edu/~cfc/Woolley2010a.pdf
The interviewer just picked 10 as an arbitrary number. The researcher just went along with it.