From the official stackoverflow account: We’re thrilled to announce we’re partnering with @OpenAI to bring best in class technical knowledge and the world’s most popular LLM models for AI development together! This groundbreaking partnership with OpenAI will drive our mission to empower the world to develop technology through collective knowledge.
That exactly tracks. You can't feed answers from an AI into an AI. It gets all incesty (technical term). So they have to ban user submitted AI answers.