Their claim will be that any of Carmack's work on those topics, because he originally began his work under Zenimax, is recreating based on Zenimax's property, because Zenimax has a claim to the early prototype Carmack thought of when pitching the ideas and there's no good way to show that Carmack isn't using the knowledge of those topics created during that period for his current work.
It's essentially writing new code looking at your old source code, and hence directly derivative of work that Zenimax has a reasonable claim to.
No no no, there's a difference from re-creating work from memory, and re-creating work while looking at an old copy.
It is also possible to work back from first principles. Heck, many Free Software was written like that: by rewriting the proprietary parts, possibly one by one.
It's essentially writing new code looking at your old source code, and hence directly derivative of work that Zenimax has a reasonable claim to.