MDM is kind of a super admin feature on Android and iOS which allows for all kinds of things normal apps can never access. It's used by companies to manage company owned devices so they can do things like reset the password and remove the apple id lock when an employee leaves.

It's not that slack can wipe your phone, but that (it sounds like) slack can detect the presence of the company MDM setup which can wipe your phone and lock you out of the app if it is not detected.