Do you have a link to the article? Was it a history article or a different discipline?

Without seeing the article specifically, it sounds to me like the author was a grad student supervisor and hired one as a research assistant. The author probably could have tracked down citations by himself, but why not give this boring job to one of his students?

Not to fill this answer up with personal anecdotes, but when I was a student, we got funding from the university, and we had to apply for various teaching and research assistant positions in order to receive the funding. So sometimes I was a TA and some years I was an RA - I worked on the "Iter" Project, pretty much doing exactly what you described, wandering endlessly throughout the library tracking down citations to compile into a huge database of works on medieval topics.

At my school there was also the Old English Dictionary project, and the Documents of Early England Data Set, and probably many others I'm forgetting. I don't remember if anyone was ever hired to help a professor track down a citation for an article, but I'm sure that kind of thing happened outside the official funding paths.