Microsoft Office doesn't have a moat but they had better product and engineers than the competition and turned that into phenomenally successful business.
I think you are overly discounting the advantage for Slack having done this for 4 years already.
Load up Office '97 on a computer today and compare it to Google docs - even 20 years ago Office was more fully featured than their most viable competitor is today (real time collaboration excluded).
Then think about all the good work that was done over the last 20 years, and how that really solid base turned into the best mobile office solution.
Office has an extremely deep moat and their continued O365 efforts are deepening it.
But it isn't based on anything besides creating a very, very featured product. Microsoft created their own moat. Same is possible with Slack. 4 years into its lifetime, one could have made same argument against MS that their word processor had no moat.
I think you are overly discounting the advantage for Slack having done this for 4 years already.