Good question. In the CC-SA clause, share alike means that the derivative work has to be licensed in the same way:
"ShareAlike — If you remix, transform, or build upon the material, you must distribute your contributions under the same license as the original. "
The complication as I see it here is that they are using copyright law, because Stack Overflow holds the copyright, and can license the content as it sees fit, rather then the creative commons restrictions.
There's also a question here if LLMs / AI models are considered derivative works.
matthewskelton