Unless part of his agreements with Zenimax required him to provide all necessary assistance and documentation so that they can patent his work, which many agreements do...
Also, they may own it as a trade secret rather than as a patented process. Or, further, it's possible that code that John is working on now is a derivative work of code owned by Zenimax. We won't know until the facts come out.
Comments
Carmack's response: "No work I have ever done has been patented. Zenimax owns the code that I wrote, but they don't own VR." [0]
[0] https://twitter.com/id_aa_carmack/status/461918500307472384
Unless part of his agreements with Zenimax required him to provide all necessary assistance and documentation so that they can patent his work, which many agreements do...
Also, they may own it as a trade secret rather than as a patented process. Or, further, it's possible that code that John is working on now is a derivative work of code owned by Zenimax. We won't know until the facts come out.