"going deeper, i would surmise that even that instruction's internal process is fully deterministic too,"
Nope. The noise source is (subject to quantum physics) nondeterministic. Data from this source goes through a cryptographic entropy extractor. For RdRand there is a DRBG that follows, to give you fast, secure random numbers and for RdSeed you get full entropy random numbers, which are limited by the speed of the source and so isn't as fast as RdRand.
Quantum physics does not answer to question as to whether or not the universe is nondeterministic. It certainly look nondeterministic but we don't know if they is actual nondeterminism or ignorance of a deeper process.
For more stuff on this, maybe check out the book in my sig.