"Your idea that it was due to his pre-existing politics could be tested if others wanted to repeat the experiment,"
Your phrasing suggests that you misunderstood my point, but does not prove it. My point is not that he started with extreme politics (or politics that might be viewed as extreme by most people since of course $MY_VIEWS are always perfectly sensible and obviously correct, it's everybody else who is extreme), my point is that he started with politics at all.
"The expected outcome of liking everything the algorithm suggests indiscriminately should be more moderation, not more extremism."
Uh, no, that makes no sense. You're engaging in an operation that is deliberately blurring Facebook's impression of you. Open your favorite image editor. Cover an image with pure white. Put a blob of black in the middle. Start using the "blur" tool. It doesn't shrink. And that's not perfect, either... the center of the blob will remain the darkest, but in this system, Facebook keeps presenting you with more "extreme" views, and you keep telling it you like it. Of course it pulls you away from the "center"... you keep telling it to. And this is where the raw volume point comes in... the extremes are louder, so if you blur like that you'll see a lot more of it.