You’re going to have to explain what you mean by centrist, because they don’t align with what centrists believe (AKA — they aren’t centrists), and they do align with the left — just not the far left.
So… is your definition of ‘centrist’ so loose that you’d put about 3/4 of the American people in that tent? Because that’s what you’d have if you put one end just to the left of Media Matters and drew a line around the spectrum that same distance from center.
Media Matters is best known for two things in terms of its policy and practices: being pro-LGBT rights, and attacking conservative and far-right media outlets such as Fox News. As a self-described centrist, I’m quite sure you’re aware that being pro-LGBT rights and anti-Fox News does not make someone a leftist. I would imagine a great many centrists are in the same boat, in that they are pro-LGBT rights and believe that Fox News is neither fair nor balanced. If you’re telling me I’m wrong and that centrists are homophobic or think Roger Ailes was a good faith, truthful television news producer during his Fox News tenure, then you are correct and we are not talking about the same idea of centrism.
Getting back to Media Matters, they do not take any actual leftist positions. That said, they are also explicitly anti-right wing, so they are obviously not a far-right outfit either. Their politics follow a neoliberal model of billionaires, corporate elites, and heirs donating to a media outfit that pushes a narrative that those billionaires, corporate elites, and heirs want pushed. Given that, Media Matters could best be described as corporatist. Corporatism is inherently hierarchical and is classified as a form of conservatism; you simply cannot be a corporatist propaganda outfit and be a part of the left. Those things are mutually exclusive.
Regarding your 3/4 number, I would say that’s a little large. Consider, for instance, that a recent Morning Consult/Politico poll found that 24% of respondents consider Fox News to be “very credible.” This means about 3/4 did not consider it “very credible.” Needless to say, virtually nobody on the left considers Fox News “very credible,” and it seems highly unlikely that many centrists do either (especially considering there is a “somewhat credible” category for right-leaning centrists to select). Given that, my speculation is that 3/4 approximates the number of people who are either leftist or centrist.