Hacker News: is closed source is a walled garden requires a "Y Combinator" account to communicate
Most people don't use software because of the driving philosophy behind it — they use it because it solves a problem of theirs.
If a tool is the best one for the job, you should use it. And if Slack has a substantially better UI for communication, maybe developers of IRC clients should try to build a better UI rather than complain about adoption of Slack.
In all fairness, while the currently running incarnation of HN isn't open, earlier versions of Arc and hn/news (along with forks) is open source, eg: https://github.com/arclanguage/anarki/
And there are clones, like lobste.rs:
https://github.com/jcs/lobsters
Then there's the API, that allows anyone to export data. So while I get your point, and agree with it to a certain extent, it's also not entirely fair.