how do you know that ? take SilkyPix (non OEM version, but what ISL sells) - how do you know that SilkyPix which is OEM for Fuji for example is not using Fuji's data... please do not generalize Adobe on others.
What part of proprietary metadata isn't clear? Even if the data isn't proprietary, doesn't mean it is useful or can be used. I can save out the metadata instructions of edits which Lightroom and ACR can share and use. It's in English, I can see Vibrance +12. But no other raw processor uses the Adobe engine so Vibrance +12 is useless. Even if that application had a setting called Vibrance, and even if it set itself to +12, the engines are proprietary and different.
DNG doesn't remove anything that stops any processor that correctly reads that data to render the raw data using it's engine.
so unless you checked (or probably looked through DNG SDK code) you did not know...
My personal comment is, so what, it's not the least bit important to me. And if it were, then yes, I'd be looking at the SDK (assuming I could do anything with out, something an engineer of a raw module should be able to do).