It goes without saying that you have more leverage before signing a contract than afterward, and the contract is obviously open ended... I was just pointing out that it's a risk of the unknown that would be weighed against compensation by both parties so it's not totally clear to me that Zeni is out of line yet.
I would assume Carmack did the usual programmer thing and just expected them to not be huge unreasonable assholes about future situations after his ZeniMax employment had ended.
This is a common mistake programmers make when dealing with lawyers and other corporate types who stand to gain money from being huge unreasonable assholes.