CoreAudio probably deserves most of the credit, there. Similar ASIO-style solutions like JACK, DirectSound and now Pipewire hit the sub-30ms mark without any big scheduler tweaks.
IOKit was designed to support CoreAudio from the start, cc gvdl.
CoreAudio was developed alongside xnu / IOKit for Mac OS X, so it’s kind of all of it. Apple had the opportunity to start fresh with a built-in super low-latency audio subsystem at the turn of the century, and they took it.
Audio actually runs on a dedicated realtime thread. This used to be scheduled differently, but nowadays it might be implemented by the FIXPRI bucket described in this document.