You excluded them from trust list and it seems they said they will exclude you from their trust list.
It takes a few seconds to exclude someone, why would he say he would do it ASAP when he could have done it before sending that PM?
Maybe they will "tell their friends" to exclude you too?
This makes more sense, but it still does look like a threat which indeed he must have said out of anger and he wouldn't really try and do anything over this DT drama b.s.