The update clearly says that they will be using some sort of tracking (likely cookies) for non-registered users and customizing content for them, as a trial.

The poster worried publicly that this would mean too many stories about Hillary on the front page -- I read that to mean he's a conspiracy nut, and doesn't want Hillary to be in office in the US, and thinks reddit could be used to push her there with this technology.

This seems like responsible disclosure, and so very far from the level of customization many content sites do (have you ever seen an ad follow you to reddit? I didn't think so), that I'm kind of surprised it's a big deal.