The bullet points for using Slack basically describe email (and distribution lists).
It’s funny how we get an instant messaging platform and derive best practices that try to emulate a previous technology.
Btw, email is pretty instant.
I get it, email accomplishes a lot. But it "feels" like a place these days for one-off group chats, especially for people from different organizations. Realtime chat has its places and can also step in to that email role within a team. All my opinion, none too strongly held.
If you work in a team, email is limited to the people you cc: while a convo in a slack channel can have people you didn't think of jump in* with information.
See the other point in the article about discouraging one on one private messages and encouraging public discussion. That is the main reason.
* half a day later or days later if you do true async, but that's fine.