The advantage Slack et al have over email is that the list of people who can reach me through them is limited. Any idiot on the internet can email me, only people we've specifically admitted can Slack me.
The author also seems to exist in a world where group chat is only used for chit chat and cat pictures. If that's the case, You're Doing It Wrong. For me, Slack is the place where we communicate project state, hash out ideas, etc. That's the value.