My guess is that it's not complete or stable or qualified enough for them to publish it as a supported feature. Maybe the power draw is too high and it triggers thermal throttling too quickly, similar to early Intel AVX-512? Maybe they are still sorting out how to have each process choose SVE vs NEON mode, so libraries using the other one don't cause a fault? (Note that this seems limited to M4 and newer CPUs.)
I agree this is an important feature. I have very often wished that my Mac had wider vectors.
Maybe they were waiting to announce it until it's been supported for a while?
Apple shipped their “Neural Engine” in lots of iPhones before publicizing or fully documenting it, could be part of their playbook.
Besides Apple who is doing hpc on Apple silicon