When scientists want to publish their results, the paper first goes to one or more referees (usually anonymous), who advise the editor about the quality of the paper. It is common for referees to ask why the original author(s) did not consider a particular question as part of their work. The reply in the cartoon is commonly used by the author to defuse that question, essentially saying we didn't consider that question because it's too much work, and just publish what I've done already, dammit.

Science is like going down a Wikipedia rabbit hole. There's always more things to do and more things to check out. At some point you just have to draw the line and say that enough is enough. Other scientists are likely to ask why you stopped where you stopped, and so saying that "it's outside the scope of the paper" is basically the nice way of saying that you stopped because you felt like it