Smart people do dumb things, especially when they think they know better than things like the law. I do think that John Carmack has done some really dumb things even though so many people are putting him on a pedestal and calling him a genius. Sure he knows code and he founded a successful business, but that doesn't automatically translate into him being smart at everything.
As I understand, Zenimax were originally going to sue Carmack for breach of his employment contract as part of the Oculus lawsuit, but this was left out. The Oculus lawsuit did establish a few key facts: Carmack did copy information before he left Zenimax, he did deliberately destroy evidence after Zeinmax filed the lawsuit, and he lied on an affidavit denying he wiped evidence.
While Zenimax weren't able to convince the jury of all their claims, it was established that something was taken by Carmack. Ultimately, Carmack wasn't directly on trial there, merely whether Oculus had clear benefit from his actions.
I have no doubt that Zenimax will counter-sue for breach of contract, theft of intellectual property and breach of NDA (among other things). Likely they will argue that Carmack's breach of employment contract is tied to his final contract payment and that damage from his actions equal or exceed the $22.5 million they would have owed him.
Carmack says that Zenimax's refusal to pay is based on 'sour grapes', but I think Carmack's lawsuit is being filed based on 'sour grapes' too. I do recall him making a rant about the independent forensics after Oculus lost the case. Now I believe that this latest lawsuit is another dumb thing being done by a smart person.
He is kicking a legal hornet's nest, especially when Zenimax's lawyers have plenty of information already gained from the Oculus lawsuit. They will know exactly what to go for when they file for discovery in this case.