The "echo chambers are a myth" statement seems a bit disingenuous to me, and misses the increased algorithmic exposure to hate in social feeds that we otherwise would not see offline.
For more reading on this I suggest “Breaking the Social Media Prism”
Also note: research shows anon accounts tend to result in more nuanced and less partisan discussion.
Online echo chambers, for better or worse, collect those of a similar interest and/or ideology from around the world. As with, say, Hacker News.
That may be benign, or it may not be. The inspiration and action are what matter. But the end result is that groups that would have no capacity to form and organise through physical contact, and which would be only very weakly integrated through print or broadcast medium, can emerge online. Again, that might be a positive or a negative development.
But what it isn't is either equivalent to IRL associations or a continuation of previous patterns of organisation.
And to that extent, Petersen's claims are grossly disingenuous.
I did an experiment to make the algorithm think I'm left wing on alternate accounts.
Not to much time passed before my feed was filled with the most hideous and stupid things some "right wing" people did.
It is the same on my primary accounts but the other way around.
No wonder we hate each other.