Well, unless IRD expects you to run a Shimano MTB rear derailler with a JTek Shiftmate, they must have either tested this combo and found it to work (the combo being a 34T cog and Campy derailler) or they are selling a product that has no "standard" application. I find it hard to believe the latter.
Obviously I have no personal experience with this combo though. You might try emailing/calling them to find out what's the deal.