Has someone actually found out what instructions do what? I assume we won't get to play with them ourselves unless they get the Microsoft treatment over the hidden APIs
There may not be hidden APIs. They could only use those custom instructions inside public functions that everybody can call, such as those in the Accelerate framework.
I consider the hidden instructions to be at least close enough to being an API to not bother using another term (AFAICT legally ISA patents aren't considered APIs)