Async communication requires an intentional approach to be successful. If you treat slack like an office cooler or a replacement for meetings, then of course it’s going to feel synchronous. The authors assertion that the tool isn’t async because it it supports a synchronous workflow (which most companies actually are set up for and understand) doesn’t make much sense to me, since async is more of a mindset than a tool.
The best approach I have seen is ensuring there are operational channels and making sure that everything goes in there. I’m personally not a fan of threads in slack for an async org since it can really fuck with the timeline and it’s more difficult to have a temporal view of what happened for a particular subject.