Ever-Infamous Kojiro-san
And technically manga can also be considered a style of art, no? Calling it a genre, yes, is silly but it's an established art style as well, I'd say.
Well, if you called "anime" a style, I'd have less basis to argue with you. There are exceptions, but there's pretty much an established art style used in Japanese animation.
"Manga" however comes in a much wider variety.
A lot of it is in "anime style," but a lot of it is also heavily influenced by American comics books and comic strips. There are Japanese comics with art that borders on photo-realism and Japanese comics made with stick figures! I'm sure you've heard of Astroboy, Crayon Shin-chan, and Fist of the North Star. And the writing styles are just as varied! A high-school romance comic's not told in the same way a basketball comic is told in.
I'll agree that manga has a common language of comics that's different from the American language of comics. But putting a sweatdrop on Donald Duck wouldn't make it manga in anyone's eyes.