TFA says "Therefore, we argue that the POSIX era is over, and future designs need to transcend POSIX." So they're actually not answering their own question, they're saying "the answer should be x" which is subtly different. There are a lot of should-be's in computing.
Ironically, while I am an heavy email user, at work it is mostly used for internal newsletters, all the communication happens over Slack.
slack replaced your internal, HR monitored, internal IM. Not email. Just so happens that lots of places were using email as their IM solution.
For a lot of these services email is optional, and even with an email they won't let you sign up without confirming a mobile phone number.