There's a big difference between a forum where every visitor sees the same front page (more or less) and something like Facebook where every visitor sees a list of things curated just for them.

If you are only exposed to points of view that you already agree with, over time it's easy to forget that some people have other points of view (which are actually not always completely ridiculous, despite what you may have heard). Note that rage bait does not count as a divergent point of view; it's almost always a cherry-picked caricature, not anything close to a good faith representation.

This might not sound like such a big deal, but do that for billions of individuals and you have the perfect recipe for the political polarization we are currently enjoying.

It's less of a difference than you might suspect. The personalized curation is in which forum you choose to visit. HN is quite niche by numbers; it's a lot smaller than say bodybuilding.com or even AR15.com. That you choose to go to news.ycombinator.com is your personalization, and also skews your view on the world. Most of the world is interested in very different stuff.