People who haven't read comics don't know about comics, there's no way to get around that, you can know who Batman, Green Lantern, Superman, Wolverine, Spider-Man and Punisher are, but that is very different from having read the comics of all those characters and noticing the recurrent patterns, the characterization and the tropes that appear in their corresponding stories, if you read Watchmen without having read comics, you're gonna miss a lot of essential details about Watchmen.
Comics are just another form of illustrated storybooks. Since childhood, at the first learnings with reading about "Jack and Jill" were done with illustrations to show the actions they were doing according to the words. Classics such as "Winnie the Pooh" or "Charlotte's Web" were all all illustrated. By the time I was seven, and reading Charlie Brown comics, I understood the comics and their recurrant patterns.
Also from childhood, superheroes like Superman, Batman, Spiderman, et al., are all part of pop culture even if never read as a comic. Their history, purpose, powers, themes of good vs. evil, are all well known, and can be extrapolated into other storylines with different superheroes in simular context. Movies, TV, and other media have ingrained the superhero into the culture, and even myths about heroes have been with us since antiquity.
I found this article as an intellectual exercise about something that doesn't apply to anyone. Any reasonably educated person has understood the nuances of the comic format, and the recurrent themes of superheroes from popculture since early in childhood. Of course, if their not reasonably educated, then why would they care to read the "Watchmen" in the first place? Like I said before, you don't need to learn how to read a novel, or know about 18th century whaling, to understand Moby Dick.