Are you sure Shimano makes that claim or was it someone else like your bike shop guy? AFAIK, once past 8-speed (7400), any Dura Ace shifter is compatible with any other Shimano rear derailleur model of nearly any "speed" era from 7-speed onward. One other exception is the new Dyna-sys 10-speed MTB rear derailleurs that aren't compatible with any Shimano road derailleur or 9-speed or earlier MBT rear derailleurs.