#Yes, this is related to community building!
The community of a blog is the blog's loyal following. Whether or not that community is directly associated with one distinct service (such as the blog's website/forum/comments section) or spread throughout social media has little bearing here - the community exists and the staff communicates with it through the blog.
Finding new content creators - preferably even from within the community - is very much a community building measure. It's about evaluating your existing community, it's about reaching that community, it's about finding outside people that would harmonize with that community.
Where does it end?
This is the big question here, I believe. Fundamentally, I agree with @AJHenderson that the line should be somewhere between "directly affects the community" and "happens to be related to a community".
However, I think the cultivation of the community, even with the one-way-communication of a blog, should be well in our scope as well. Into user retention flow a lot of factors - many of which may be entirely without communication between staff and users. A user may leave in response to a staff message they read, without ever saying a word.
"Product development", as you phrase it, is more about the content. The example question is about a meta action - getting to the content - not about the content itself. We don't need to know how nicely the author can describe the local paper factory's colours. We do care about how they interact with the community, possibly their history, or even some other factors that relate to staffing. These are all very important to building the mentioned loyal following - the community of a blog.
But more importantly for this example question is that it asks for methods to find such authors. Interacting with the local community, representing it elsewhere, and interacting with other communities - that all falls under that umbrella as well.
To sum it up, I do think that the line between product development and community development is rather fine and we might get some edge cases in either direction, but the overall distinction seems rather clear.