I'm no lawyer, but I'm sure the IP belongs to the company and not to the employees, so it's not "inherited" if some of the staff leaves and creates a new company. They might use the know-how of the tech to create a new, similar IP, but that's going to be a new, independent IP. (If it's not, or too similar, then the old company can sue them.)
Again, the IP did not belong to the staff, but to the company as a legal entity, and Illusion has no legal successor for sure. On the other hand by releasing HC, now IllGames (the company, not the staff) most certainly has the IP rights to all tech used in HC (which, as we all know, almost the same tech in practice, but not 100% the same).