They definitely do have OSX running on ARM64, they had OSX running on x86 for years before the switch (in fact they had OSX running on x86 before it even was OSX, NeXT ran on x86, SPARC, PA-RISC and 68k, PPC is the one Apple had to add), they've already gone through two architectural migrations (68k -> PPC and PPC -> x86) and by all accounts the iOS core is very much shared with OSX, it wouldn't make sense not to port OSX along the way.
Although NeXTStep ran on x86, the MacOS build on x86 was John Scheinberg's personal skunkworks project until it became Marklar in 2001 (and then kept under the hood for another four years). Mind, Darwin was always written to be portable, but it wasn't a deliberate strategy to take it to Intel. This time around though, I too reckon that they have already a MacBook running on an A10X in the labs.
I was just listening to a Windows Weekly podcast about that, and its limited to 32-bit x86 binaries only, no 64-bit support. They also said its "unusable" for 32-bit apps (in particular Chrome), because you can watch the system drawing the windows of x86 apps on the screen.
Watching the video, they seem to be exaggerating a bit with "unusable", but Chrome does look sluggish, and the startup of "DrRacket" and its window redraw does look very slow too.
Edit: After watching the video, that didn't seem too bad. Perhaps it would work well enough on top of a beefier ARM processor? Of course the lack of x86_64 support is another issue that may not have a reasonable solution.
I'm reminded of the patent-sharing agreement between the two. Or, given MS' diversification, they may be willing to directly license the tech. Making the x64 -> aarch64 translation as robust as possible has benefits for both companies, and they're not nearly the bitter enemies they used to be.
No, they have tech for executing i686 binaries on aarch64, not x86_64. Big difference.
Meanwhile, High Sierra is the last macOS release to support 32-bit binaries.
"Without compromise". We'll see what this means later, but most likely it'll mean that macOS won't ship with a 32-bit runtime and you'll be able to download as you do with Java.
Or it could be nothing. This is a pretty thin article.