The announcement came from GBATemp.net: someone by the screenname shutterbug2000 got Windows 95 running on his New Nintendo 3DS, and he provided photos and video evidence to prove it.
Others were soon able to accomplish the feat per his instructions. Folks have gotten emulated games to play on Nintendo consoles before — at least for a week or so before Nintendo patched the exploits they found — but this is the first time en entire operating system has been made to run on one.
Emulation is difficult, and often requires significantly more power than the device being emulated (eight times is a number I’ve heard thrown around). This is likely why the newest model of the 3DS is being made to run an operating system that’s twenty years old as opposed to anything close to current. Right now loading Windows is the extent of how far they’ve gotten: they won’t be able to load ancient PC programs or use any of the built-in features until they tinker with it further.