Like in Animal Man, Grant shows up, and tells Animal man he is a comic book character, but considers him "real" cause his adventures are in print, and therefore, exists in one shape or form.
He and Alan Moore are "magic users." Grant goes as far to write down spells in his work (Like Invisibles) "Just to spice things up."
With Alan, he believes magic is not so much a belief system, but a matter of effort and standard. For magic in the basic form was called "The Art."
So anything, can be magic, with effort effort, and talent.
Edit: BTW, Grant, Mark Millar, and Warren Ellis all come from Scotland, Alan, came from a really, really backwoods English town that was pretty close to Scotland. Kinda ironic they have such a huge impact because for them America was a big impact on them.
King Mob in Invisibles is supposed to be the Author Avatar of Grant himself. (He is a bald white guy.)
And I thought of a better way to describe Grant's "Chaos Magic" thing.
It is like the Matrix sorta. In in that reality has "code." In Chaos magic, you use your will power, beliefs and traditions to mold, break, or alter said code, to make magic happen.
Alan Moore's best work of magic is Promethea. I highly recommend that, and i love the Invisibles btw.