Google does an insane amount of very deep work all the ways down to the compiler level getting Chromium compiling on Windows and compiling on different architectures. These teams are very well staffed.

It makes strategic sense to go with Chromium if they're aiming for ARM64 support if they can get a huge amount of work for free directly (as opposed to incidentally) from Google to support clang on Windows – which is a much bigger undertaking than it seems at first glance.