A lot of people who read comics also make them. Maybe even most people.
Almost everybody I’ve ever met who reads comics has, at some point or another, actually made one, even if he or she never showed it to anybody.
To me, this has to do with the nature of the visual language of sequential images. Unfortunately, our failed definition of "comics" conflates the idea that "comics=sequential images", when really "comics=cultural context" and "sequential images=visual language." By this notion, what Joey is talking about is that comic readers are highly likely to create stories (comics) using this visual language.
Perhaps this should be unsurprising then, since it means this visual language works like any other language. People get exposure to it, and imitate it in order to learn and practice their fluency.
This is not a case of people "drawing comics" the way that filmmakers "make movies" or other types of "artistic" craftsmanship. Rather, this is more like the way that Americans "speak English" or Quebecois "parle en Français" because those are the languages of the communities to which they belong. Comic readers constitute the language group for visual language in America, so their "drawing comics" is simply participating in the (visual) language of their community.
Note that this also applies to the particular graphic dialect that they might partake in: those who read manga are likely to draw like manga, those that read superhero books are likely to draw that way.