The fact that these researchers focused exclusively on developers in a single ecosystem (the R language) is, in my mind, a death blow to the generalizability of their study. That is, their conclusions might be relevant to R developers, but not to open source developers in general.

We all know that different ecosystems typically have different values.

But more importantly, R developers are particularly non-representative of the more general open-source ecosystem. Based on anecdotal evidence, I believe it's highly likely that academics and people who primarily identify with non-CS fields/disciplines are over-represented in the R community when compared to other communities.

(Note: At least within the text of the paper, the researchers DON'T claim that their results generalize to all OSS developers, and don't even claim their results generalize to all R developers!)