If you were talking about using these protocols over a telnet session, then I would be more inclined to agree with that statement, but just installing an application, entering a server name and account credentials (the latter of which aren't needed for IRC), and connecting isn't any more difficult than creating an account on slack and connecting by entering the address in the browser.

Regular users have amazing blind spots. Also you’re not giving slack enough credit for their smooth onboarding process.

Of course, but I don't think that Slack has done a better job with this compared to previous solutions. In a lot of ways, it's worse given its performance issues. For example, I never experienced UI lag in a chat application until I used Slack (comparing it with UIs like a GUI IRC application, AIM, ICQ, MSN messenger, Yahoo messenger, Skype, etc). Second, auto-completion and search do not work like they do in previous chat applications (or other applications in general) due to infinite scroll and the default of doing a global search instead of just limiting it to the chat or group. Third, the fact that you're forced to join some channels because you were invited or can't leave a channel because you're the last one in it are other issues that come to mind.

As much as it annoys me too, it appears that people actually using it have enough tolerance for it that it doesn’t matter.