But they're also prioritizing news sources with such snippets, which forces publishers to have them or lose traffic to competitors, because Facebook is taking advantage of a dominant position in the market.

That's what they're arguing about, it's literally the same thing that happened with Google news (and was negotiated) years ago.

Are they putting a thumb on the scale to prioritize them, or are users organically deciding that they are more interested in them? The former I agree would be concerning, but I hadn't heard of this happening before and the article doesn't seem to engage in this level of detail.