For example, if all members of Congress were to shout loudly when a particular member got up to speak, drowning out their words, would this be censorship, or just their exercise of a community vote against that person?

One thing to note is that your comment wasn't removed; it was collapsed. It can still be viewed by anyone who clicks the expander or has their threshold set sufficiently low (with my settings, it's expanded). There is a tension between the threat of censorship being a problem on the one hand, and the ability for a community to collectively decide what they want to talk about on the other.

The censorship issue is also diluted by the fact that 1) nothing here is binding on anyone (which is way different than your Congress example), and 2) there are plenty of other places people can discuss things, online and off. It is still somewhat relevant, of course, to the question of whether there's an echo-chamber effect, but carefull not to pull in additional connotations with choice of words and examples.