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.
The technology that article mentions is Microsoft's Intel->ARM translation, which is surprisingly good, and entirely unrelated to Firefox.
The "optimized" build they're talking about is almost certainly natively-compiled ARM64, and the reason it's hard is because Rust is built on an older llvm version...which doesn't have Google's patches that improve support for Windows ARM64.
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.