It’s truly got some of the worst cgi I’ve seen in a big budget movie in a long time. The babies in the beginning looked cursed, the time travel sequences looked like they were made up of wax figures, Batman’s movements sometimes looked like they came from a late 00s video game cutscene, Zod’s face looked like a bad deepfake most of the time and all the cameos at the climax were so badly executed the cgi was more noticable to me than the cameos themselves.
I think they’re feeling optimistic about it if they’re including him. Otherwise I can easily see them replacing him and inserting the new actor into the already shot footage, like how Ridley Scott did when he replaced Kevin Spacey in All the Money in the World.