In this case, and others like it, a clean room is challenging because a principal (in this case Carmack) left one company and joined another. So the 'taint' if you will is in his head.
1) The IP in question was only reimplementable with Carmack involved, and the lawsuit was basically inevitable. Hiring Carmack in this scenario is reasonably perceived as an end-run around buying/licensing the IP itself.
2) The IP could be reproduced clean room without Carmack involved. Lawsuit is easily headed off.
