I think to a lot of people who pay attention to this sort of thing (eg the crowd already familiar with tools like reveddit) might not be a shock but it is a rare case of it being admitted in clear terms with specific examples which correspond to the sorts of claims that are often dismissed as rumors by third parties. While not anywhere near as high profile, it's kind of like how anyone who was paying attention probably had a decent idea of the kind of surveillance governments might be engaging in, but it was still a big deal when Snowden released the documents making it undeniable.

As the thread says, Twitter used a bit of trickery by calling it "visibility filtering" so they could lean on the technicality that "we do not shadow ban", because they too preferred to leave in some plausible deniability.