xkcd

I think there's a difference between being able to understand and having all the information to understand. Randall has a very wide range of topics that he makes comics on and most readers won't know enough about every topic to get every joke, even though he does his best to simplify it for a broad audience. That's the reason ExplainXKCD exists. But his audience is generally nerds, people who like learning new information, which is also what Explain caters to. So it is both true that his audience can (and are willing to) understand and that Explain helps them understand. (I also think that nerds love explaining stuff, so it makes sense that the main fandom website is ExplainXKCD, even if there were no demand for explanations.)