I don't see any mention of x86_64/AMD64 in the article. I would expect a version of Windows which can't run x86_64 apps to result in a lot of nasty compatibility surprises--it's been awhile since developers had to support 32-bit-only hardware--so I'd expect Microsoft to support it if at all possible.
You're right, the posted Anandtech article does not mention x86_64; however, other prior and recent coverage of this topic does confirm that x86_64 is not supported, such as this Ars Technica article [1] from the same day.
There's no support for x86_64 binaries. Windows 10 is still available in 32-bit editions and almost all apps are still available for 32-bit, so I don't think this is a big issue.
I assume any kernel drivers will need to be compiled for ARM64 though. This might not help corporate adoption--VPN client, 3rd party anti-virus/full disk encryption, etc.
Also one thing that is x86-64 only is WSL, the Windows Subsystem for Linux.
I think most Windows applications which are truly 64-bit only are either too large to run on a machine like this, or likely to be rebuilt for AArch64. For the time being, I think IA-32 support is enough to tide them over, and it has the benefit of being thoroughly out of patent (except some fairly common extensions, like SSE2, which will expire soon).
There's still a lot of Windows devices that run x86 Windows, and new devices still ship with it, mostly low-memory or low-disk-space devices. Thus apps need to support it.
