I’m thinking this is related to the machine-learning coprocessor. They likely expose this to developers through their ML API and don’t want to have to support this for external customers if it changes. Still this is a great find!
While AMX could be used for deep learning in a pinch despite the lack of support for common formats like fp16 and int8, I suspect Apple had some other use cases in mind as well. For example, 64-bit float support is expensive and generally useless for ML. However, they are useful (though not necessarily required) in problems such as bundle adjustment that may appear in the context of frameworks like ARKit.
Looks like apple has decided to make it very tightly integrated into the CPU. It means while doing ML operations, the CPU core can't go off and do other useful work. It also forces all cores to require matching ML hardware (or have a lot of OS complexity as certain threads can only run on some cores). The benefit is latency to get ML stuff done is lowered from microseconds to nanoseconds.
I suspect Apple probably messed up with that tradeoff... Very few applications can't wait a few microseconds for results of some ML computation...
The tradeoff is not likely perceptible to developers since the compiler or runtime will decide if the net is small enough to run locally or whether it should be dispatched to the dedicated device (again, like L1 access for the majority of developers not writing inline assembly) - thus why it's not exposed. They (apple) could also do some very interesting things with it internally such as neural branch prediction and cache eviction based on contextual operations - again outside of the scope of what a developer would have access to.
Intel is doing the same in their upcoming x86 silicon with Intel Advanced Matrix Extension - I would expect this ISA to be heavily inspired by that.