I didn't read this as a slack good / slack bad article at all.

At my current job I was asked to join a client's slack group (added without my consent) and it was a nightmare. The problems have nothing to do with notifications or interruptions, but it gives them the implicit understanding that I am a custom developer for their needs. Since they can reach me at any time, they assume that I am going to fix their bugs, add their features, etc. It circumvents what I think is the proper procedure for work. I suspect that's more the heart of why the author is saying they aren't going to join their client's slack group.