No need for any "4d chess defense". There was a lot of cheap checkers cheating in the article though.
For god's sake, he didn't cut the electricity in the data center (which would actually fit the "sky is falling" narrative), he just cut access to a glorified enterprise chat app.
They can always use email, bug trackers, task management, their enterprise intranet collaboration and discussion tools, and several other things besides, or, god forbid, have teleconferences, pick up the phone, or discuss it face to face in the office.
If they can't work with those, then lack of Slack isn't the problem...