Changing the cheat flag from false to true didn't seem to do anything. I tried hosting a co-op game with your cheats.xtbl and joined my own game on my secondary computer, having the same exact .xtbl file as well. Still has the cheat blank where it should be under 'world' but its just highlighted empty space.
My original thought was that the selection to enable the cheat was still there, but something is coded to block input to actually confirm activation of the cheat. Somehow I have a nagging feeling that it isn't that simple, although having the cheat "theoretically" still able to be selected in the phone is a bit sketchy.
Perhaps they're planning DLC for a co-operative zombie mode, which is the only clear and present solution in my mind, for such a mode that one would be insane to not add in co-op.