The leak includes the kernel for the operating system of the OG 2001 console, a custom version of Windows 2000.
Making matters worse is that the code for a version of Windows NT 3.5 has also leaked online.
The debut Xbox console doesn't have much of an emulation scene, at least compared to some other nostalgic consoles, and this is partially because it's very difficult to emulate the original Xbox operating system and kernel due to the custom tech Microsoft used when creating the console.
As mentioned above, source code for Windows NT 3.5 has also leaked, and this source code is apparently for a near-final version of it.