It’s a joke about the difference between doing something for work, versus doing very nearly that same thing for fun. For example: there are many mechanics who work on cars all day, but spend their time off working on their own project cars for fun. Myself, I work in IT all day, and the go home and mess around with my home server for fun. Here you have two researchers who spend all day researching, and then are presumably going home and researching for fun, which is a silly extension to this concept.