I would be shocked if Apple didn't have macOS running on some ARM development systems internally. It's not even that much of a stretch -- both systems use nearly the same kernel, and the iOS userspace runs on x86 as part of the iOS Simulator.
afaik there is no ACPI with ARM so it's non trivial to setup an ARM system that has modular hardware as we take for granted in the Intel world. Maybe they went through the trouble and are keeping it under wraps. Or maybe not.
It started out as a hedge against their own hardware being an impediment to sales, then ended up being their entire business. I guess it was a good hedge to have!
Considering they had NeXTSTEP release versions running on x86, PA-RISC, SPARC, and its original home 68000, they did pretty well. I do wonder why the MIPS chips were ignored.