Accelerate

Apple's system BLAS, supplied by macOS and installed with it.

The one supported vendor whose single compute thread cannot be read back. It exports no thread-count entry point, so the requirement is established through ACCELERATE_THREAD_LIMIT and reported as ThreadEvidence.Unconfirmed rather than claimed as checked.

It stays selectable on that basis: the requirement is that a backend be held to one thread, not that it be able to describe itself, and Accelerate can be held. Treating an unreadable count as a failure to enforce would leave macOS with no vendor at all. The cost is that its arm carries weaker evidence than the others, which its reports say.