I don't think that's the problem. The problem is that an AI can't know truth from falsehood, or when things are being omitted, overtly emphasized, etc. The only thing it can actually evaluate is tone, and the factual, objective affect that all news reporting tends to use is gonna read as unbiased. It'd only register as biased if they started throwing out insults, used lots of positive or negative adjectives, or other kinds of semantically evident bias. You'd basically need an AGI to actually evaluate how biased an article is. Not to mention that attempting to quantify that bias assumes that there even is a ground truth to compare against, which might be true for the natural sciences but is almost always false for social reality.