I don't think that's the case on HN, which is what is being discussed on this comment chain. If you're indeed referring to HN, I'd be glad to read an expanded argument.

I agree that on Twitter this is a much more complicated matter.

Considering that downvoting and flagging have karma thresholds - coupled with the vouching mechanic for dead comments. I honestly think HN has a pretty good setup for this. We've also got something miles better than Reddit - a limit on how much Karma you can lose on a given comment. I think that works wonders against echo chambers by allowing objections and clarifications to be raised without any real fear of being karma bombed for it.