I still use Slack for internal use, and for our students.
What makes Slack (and its ilk, though the title of the post makes a bias against Slack clear) so great is that it's simple, easy, full-featured and has a low barrier to entry to the non-HN crowd that is being asked, in many cases, to use it.
I can promise you, out of experience and out of just plain statistics, that Slack and IRC have different audiences, and those audiences will not likely prefer the other.
Also, I'd guess that IRC is actually the more popular choice. But the majority of the IRC using audience is too busy getting work done in IRC to comment on an HN post.