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?
It is highly unlikely that anyone had time to talk about the sequel when the production was hugely over-budget and over-schedule to the point that it almost bankrupt the film itself and Lucasfilm along with it. And the real reason for his removal is right there; “the production was hugely over-budget and over-schedule”.