Not quite the first! I actually had a Surface RT, and for a while you could jailbreak it and run Win32 apps, as long as they were recompiled. Obviously there wasn't much available (turns out there wasn't much motivation for recompiling apps for the tiny fraction of the tiny fraction of people with jailbroken Surface RTs, whoulda guessed), but there were a few: someone actually ported the Python runtime. It worked great!
It was always inexplicable to me that Microsoft did all that work to port the entire OS to a new architecture and then roped it off and said "No touching!". Looks like it finally wasn't a waste after all.
Edit: My bad, the title specifically said ARM64. Will leave the story up as a hopefully entertaining anecdote though.
Microsoft didn't want to spook Intel too hard, that, and they wanted to motivate developers to buy into the sandbox "Store App" environment. So long as Win32 worked, why bother putting effort into designing an app specifically for Windows RT/8?
There's another reason for Microsoft to allow full ARM development with the NT OS: Windows CE is dead, and I believe many IOT/Embedded shops might be too put off with other embedded systems - and/or don't want to rewrite their ATM UIs or Factory robot arm controllers (both historically the realm of WinCE) as Android apps or "Windows IOT" UWP apps. So saying "Windows 10 now also replaces Windows CE" is a good thing, especially given the relative ease of porting Win32/CE code to the full-fat Win32/NT implementation.
I think this will only be news if we hear them announce unlocked ARM for all phones and tablets. It's still possible unlocked ARM is entirely up to the discretion of the OEM, not the user.
It was always inexplicable to me that Microsoft did all that work to port the entire OS to a new architecture and then roped it off and said "No touching!". Looks like it finally wasn't a waste after all.
Edit: My bad, the title specifically said ARM64. Will leave the story up as a hopefully entertaining anecdote though.