The argument about why slack doesn't work well is a bit of a stretch. I understand all the points on closed system etc. But the author states that slack doesn't work because it's synchronous and it excludes others from participating in the conversation. Then mentions that even with history recorded, the person in a different time zone can't participate. Then the argument is to use email threads instead. email suffers the same problem of slack, where those who didn't check their email while a discussion was happening now feel missed out on. However, they can reply 8-20 hours later or even days later. Well, you could do the same thing with slack. The problem with slack is the lack of relevant threads. Everything is one giant thread. But that is how most email mailing groups become anyways.

Now, I am not a proponent of slack, I think it has its own problems, but I don't think email is any better.