It's a good proxy for both. Both popularity and volatility have misleading statistical failure modes if measured purely by pull requests.
The author did narrow it down into what it mostly accurate represents though:
> Nevertheless, in my view, the number of pull requests is an important indicator of how much people are willing and capable of contributing to your software in the open source domain.
There has to be something to contribute to first -- i.e., new features or bug fixes. Once software reaches a level of stability, there aren't new features to be built, or bugs to be fixed. That doesn't mean there aren't contributors out there, willing to contribute to something new.
The author did narrow it down into what it mostly accurate represents though:
> Nevertheless, in my view, the number of pull requests is an important indicator of how much people are willing and capable of contributing to your software in the open source domain.