Content matter is adult, not in the sense of pornographic, but in the sense of serious and too sophisticated... for a juvenile audience... disposed towards realism (here we mean contrary to the science fiction... of superheroes comic books).
Aristotle, in the Poetics, did a far more elegant job of characterising tragedy and comedy through their content, but merely through the type of story and personalities that could be presented on stage. That this notion has fallen out of fashion - probably because Shakespeare demonstrated that the rules did not need apply for popular, intelligent and serious drama - is reflected in most definitions of art forms. Content usually defines genre, and that the pair go on to claim that the graphic novel can encompass documentaries and autobiography, even history writing.
It's a shame they have forgotten what a novel actually is, by this stage: when Joe Sacco is evoked as 'journalism' (not inaccurately), the notion of 'fiction' as a key element of the novel is abandoned.
They also get ridiculous when autobiography is identified as a possible content: Ghost World is associated with creator Daniel Clowes' personal life, quoting a generic comment from an interview about the relationships between the female characters, even Frank Miller's Batman lives in a world informed by the writer's experiences in New York.
Yes, writers and artists work from their life experiences. Using this to define the difference between two formats is trite.
Why on Earth did they set themselves an unnecessary task?