I don't agree that this particular trait of macOS makes an implementation at the layer WINE implements (full API+ABI) any harder or easier. Certainly, vertical integration makes OS porting (ie Asahi) and lower level emulation more difficult, since the hardware and firmware's contract with the OS is entirely proprietary and can be freely changed from revision to revision, but something like WINE operates many layers above this anyway.

There's a reasonable argument that the Win32 API was designed to be more stable than the Mac OS one and this makes WINE's task easier, but this sword is double-edged: while slow change is useful, backwards compatibility expands the Win32 API surface area, since each API also needs to continue to support every way it was ever wrongly used. With the continued proliferation of Windows UI frameworks and OS subsystems, it becomes difficult to argue that Windows is any easier than Mac OS would be to implement at an API layer.

I do think there is an argument to be made that Win32 was more straightforward and felt more tractable when WINE _started_, and therefore the project was able to scale with the problem space, while OS X started from a "larger" place and was therefore more daunting, but this again is a timing and traction argument more so than a purely technical one.