Slack is a tool, just like one of probably dozens you use in your day-to-day work. It doesn't set or enforce any guidelines for how it should be used. If left unchecked it has a tendency to amplify the worst parts of your company's culture. If you make an effort to moderate it, it can be a huge asset to productivity.
Speaking of the example in the article, there is absolutely no reason clients should have direct message channels with your company's employees and expect to get a response 24/7. This is true with email, Slack, SMS and every other form of communication.
The criticism I most often hear is "Slack expects me to be online all the time". No, YOUR COMPANY expects you to be online all the time. Slack can be closed with one click.