The link you provided does say "The DX4C transmitter is compatible with Spektrum™ DSMR™, DSM®, DSM2®, and Marine surface receivers." But it's in the "overview" tab.

I didn't know it was backwards compatible until I read the manual. It was a pleasant surprise, and convinced me to upgrade from the DX3C rather than go to a different brand. (Yes, I'm one of those wierdos that reads the manual before buying.)

I'd also guess that Spektrum made a special effort to nuke the clone receivers. Maybe I can sell off my old receivers to the guys that now need to replace their Oranges.