Right but that doesn’t absolve him of anything, he willingly designed an AI and punted solving the mitigation problem into the future.
It's not like designing an atomic bomb, where the entire goal is to get to a point where we suddenly have to worry about global annihilation. That's a well-defined threshold, and there's no pretending you can accomplish your goal of having such weapons without opening pandora's box.
It's more like working on genetic engineering, which can be used for a great deal of good (e.g. curing genetic diseases, making drought-resistant crops, curing certain cancers, etc.), but at some point will usher in the era of human cloning and human gene editing. Nobody knows quite when that threshold is crossed, but everyone knows that's one of the eventual outcomes. It's an outcome that can likely be mitigated through public policy and perhaps other measures, and it's not unreasonable to think that we'd work on such problems as the potential draws near.
That's the calculation he made: let's try to improve AI for all the good that it can provide. As the risks become more salient, they will get the focus and attention they deserve and we will likely also have new technologies for mitigating them. Now, he sees that the risks were closer than he thought and it doesn't help that world-wide governance has been on the decline for probably at least a decade.
There is no “fixing it in post” outside of Hollywood.
That's why we're talking about it, and why he's speaking out. What he's doing now is trying to lend credibility to the concerns others have raised, and he even quit his job because of it. There's something to be said for trying to be a part of the solution to a problem you helped create.
Let's not pretend AI wouldn't have happened, if not for that guy. Maybe he accelerated progress by months or years, through his life's contributions, but you can't pin it all on him. The ideas of artificial neural networks go as far back as the 1950's, and interest in them has never completely gone away. They fell out of favor, for periods of time, but it was almost inevitable that people were going to harness the massive compute power of modern GPUs to revisit some of those topics.