I wouldn't say it's necessarily wrong; it may be (accidental) foresight. Windows has added a UTF-8 code page, which means you can get as near enough as makes no difference full support with the A functions instead of the W functions.
That said, even now in 2024, it's not clear how much of a bet Windows is making on UTF-8 versus UTF-16.
I would be surprised if the UTF-8 support in Windows was anything deeper than the A functions creating a W string and calling W functions, which is what Rust is doing already.
The question is whether that's all they ever do, or whether gradually some parts of Windows begin to decide to be UTF-8 first and their W functions now translate the opposite way. This must be tempting for some networking stuff, where you're otherwise taking a perf hit compared to Linux.
That said, even now in 2024, it's not clear how much of a bet Windows is making on UTF-8 versus UTF-16.