I mean, could it be possible George and Gary talked about George’s evolving ideas for Star Wars III during production of Empire, and maybe Kurtz wasn’t thrilled about the direction they were taking, so he contributed that to his departure somewhat, regardless of whether he was fired or if he left voluntarily?
This seems very likely given the roles of both men and time spent together working on the films for many years. If Kurtz did send in that resignation letter from the company late in '79, it seems he carried on working on it - even in another role - and was asked to do further work/have credit for ROTJ.
Kurtz did putline Georg’s loose 9-episode idea for each film - and would have likely known them as his job as Producer. Though like for all the films by George - they changed a lot, and later changed more to fit George’s ‘I had it all planned out’ plan 😉