With all due respect, I think this information about SDK capability is not entirely accurate.
I'm a developer as well and I have some C++ background. Just for curiosity checked the header file (PMDG_NGX_SDK.h) available on SDK, actually anyone can check it on PMDG installation folder.
I'm not saying what is or not possible, but for sure there are several references to CDU on this file (147 matches), IMHO read and write variables to CDU inside SDK.