he's basically correct, dx10 wont be backwards compatible. Older version of dx were backwards, but looks like ms is straying away from that. More than likely other dx calls will be mapped to dx9 (emulated via software layer). So, it is true but at the same time there are exceptions.
If you argue with an idiot he will drag you down to his level and beat you with experience.
I am not a fast writer.
I am not a slow writer.
I am a half-fast writer.