There's a lot of missing information, but I consider the most plausible case is that Carmack produced useful VR knowledge while working for ZeniMax. ZeniMax wasn't interested, but even so, they legally owned the knowledge that Carmack created while in their employ.
For high-level programmers, the line between company-time and personal-time is pretty gray. I've started several projects on my own time, seen if my employer at the time was interested, and then proceeded on company time if they were. However, I was aware that if they weren't, I couldn't simply use the code I produced while in their employ elsewhere (except for specific exceptions I had written into my employment contract for pre-existing projects.)
It's an unfortunate situation, akin to people getting in trouble for giving away equipment that the company was going to destroy. Yes, it was going to be destroyed, but the employee still doesn't have the right to dispose of it as they see fit.
The language of theft, etc. is strictly media-bait. However, I strongly suspect that ZeniMax has a strong legal claim to at least some ownership, as witnessed by discussions involving equity before the Facebook takeover.