If someone came to you and said "good news: I memorized the code of all the open source projects in this space, and can regurgitate it on command", you would be smart to ban them from working on code at your company.
But with "AI", we make up a bunch of rationalizations. ("I'm doing AI agentic generative AI workflow boilerplate 10x gettin it done AI did I say AI yet!")
And we pretend the person never said that they're just loosely laundering GPL and other code in a way that rightly would be existentially toxic to an IP-based company.
The reality is that programmers are going to see other programmers code.
Content on StackOverflow is under CC-by-sa, version depends on the date it was submitted: https://stackoverflow.com/help/licensing . (It's really unfortunate that they didn't pick license compatible with code; at one point they started to move to the MIT license for code, but then didn't follow through on it.)
So, for the specific case of material contributed to StackOverflow on or after 2018-05-02, it's possible to use it under GPLv3 (including appropriate attribution), so any project compatible with GPLv3 can copy it with attribution. Any material before that point is not safe to copy.