I can't see how this deal is in anyone's interest.
OpenAI need high-context, unpoisoned data to train on, and even if they correctly keep tabs on what responses are AI-generated, the effect will be to reduce the amount of human-generated expert replies.
Stack Overflow's "product" isn't answers but expertise. If OpenAI could replace that, why bother with StackOverflow, and if not, Stack Overflow just put a big question mark behind their main selling point.