I think they vastly underestimate how many things Meta tracks besides ad tracking. They’re likely tracking how long you look at a given post in your feed and will use that to rank similar posts higher. They know your location, what wifi network you’re on and will use that to make assumptions based on others on the same network and/or in the same location. They know what times you’re browsing at and can correlate that with what’s trending in the area at those times, etc.
I have no doubt that their algorithm is biased towards all that crap, but these kinds of investigations need to be more informed in order for them to be useful.
They’re not that different any longer: https://news.ycombinator.com/item?id=36770883