Jonah, one of the first things you learn in the media-crit biz is don’t say or even imply “never” unless you can prove it. It’s true that Media Matters for America will on occasion upbraid the Washington Post and other liberal outlets for not being liberal enough.
But your writer is correct on the general point: the MRC tries to stick to pasting supposedly “objective” media outlets and avoid picking on opinion journalists — columnists, talk radio hosts, and so on. Media Matters spends the vast majority of its cyberspace on the openly conservative media. They are not interested in pursuing objectivity — that would be a net loss to liberals. They are interested in knocking down the conservative media before they can knock down the liberal media.