I think what he means is that if you take the entire article as a bag-of-words model, the general topic of the article will be a stronger signal than the position the article takes with respect to the subject.

The latter is a lot harder to extract from a bag-of-words. In fact the only thing I can imagine is if an article uses a lot of euphemisms or negative synonyms of a topic.

So you get the factual reporting articles regardless of left/right bias. With the more polemic hyperbole using articles, it will filter a bit more of the angle you disagree with, which adds some bias, but it also good for your blood pressure and a polemic article you disagree with is not going to make your view more balanced either.