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Moving item on list bound to CoreData

I came across a code snippet that semi-explains how to move items in a list bound to CoreData. What I don’t understand is how it is working since map would return a copy and not a reference to the original array. Aside from saving, it’s never updating the fetched results data.

    func move(from source: IndexSet, to destination: Int) {
        // Make an array of groups from fetched results
        var revisedGroups: [PrayerGroup] = self.groups.map{$0}
        // Change the order of the groups in the array
        revisedGroups.move(fromOffsets: source, toOffset: destination)
        // Update the order attribute in revisedGroups to persist the
        // the new order
        for reverseIndex in stride(from: revisedGroups.count - 1, through: 0, by: -1) {
            revisedGroups[reverseIndex].order = Int16(reverseIndex)
        try? moc.save()