Author here.

I have never used IRC in a company, so I can't compare. Slack is the topic, because we used Slack. If we have had used IRC or HipChat or Microsoft Teams, then maybe we have come to the same conclusion.

Yes, summarization is important. However switching to E-Mail causes all its problems. New colleagues don't have the mails for example. Basecamp has chat and a forum/message board or todolists. We can easily link between these.