It's a huge mess, welcome to code licensing!
As it stands right now content on SE network is indeed under Creative Commons with all the implications.
Some people grant additional permissions to their code contributions, for example I explicitly license all my code here under MIT as well (by saying so on my profile).
Obviously Stack Overflow existed and had this problem even longer than WPSE too. So this is basically elephant in the room, the world of programming is choke full of code originated there. Alas, that gives more responsible people (and lawyers I guess) a fit.
So my personal approach — if you want to use substantial (I know this is vague, I define this for myself as considerable chunk of code, implementing some functionality) code from SE then ask the author to license it fittingly. I did this multiple times and never was denied.
Tiny logicless (or nearly so) snippets, you could write yourself quickly, I think are fair use and fair game. If someone magically enforces licensing on all of that — programming industry will collapse overnight. :)
If you want bulletproof certainty just ask for license for everything.
On the SE network level there are now plans to switch code license to more permissive. Guess what — it's a huge mess. See A New Code License: The MIT, this time with Attribution Required
As a followup question, does any user who posts WP code samples on this site violate the GPL by re-licensing their GPL derivative code under CC BY-SA?
If you mean WP code in general — the GPL derivative stance of WordPress project is more their wishful thinking than a real thing. Official plugin and theme repositories themselves do not enforce this.
If you mean GPL code from elsewhere (like WordPress core) used here in answers — it falls under fair use (more or less).