This seems to have a pretty serious reverse-causation problem -- it's possible that popular software gets more contributors because it's popular, which seems intuitively correct, and that explains the correlation between number of contributors and popularity much more clearly than this nebulous idea that "maybe the number of contributors makes the software better and better software becomes more popular"
Yeah, that's why that intro section is only demonstrative speculation, with explicit claims of no causality. The actual analysis is completely independent of that.