The imported post is CC BY-SA 3.0 because that's how the source (Stack Exchange) licensed it. That notice reports a license.
The list of licenses you can choose when posting here is unrelated. That list is configurable (per-community), so if people want 3.0 to be an option we can add it. So far, the feeling has been that 4.0 is better than 3.0; I haven't heard of anybody explicitly wanting 3.0 over 4.0 yet. But if someone does, we can add it.
You might be remembering the outcry on Stack Overflow (Stack Exchange) when they changed to 4.0. The issue there wasn't the content of 4.0; it was how the company made the change. Initially they simply declared that everything that had been contributed under 3.0 was now 4.0, which was a violation of the 3.0 license. People did not appreciate having the terms of their contributions changed out from under them without their consent, so there was an outcry when the change became known. Something like a year and a half after they made the change, they fixed it to restore the original license to already-existing content, applying the new license only to new posts.
Codidact lets the author choose the license from the start; we don't think we should be changing terms on people. For similar reasons, we don't allow people to change the license after posting; content, once offered under certain terms, shouldn't be changed to a more restrictive license. We could, in principle, say that you can move from a less permissive license to a more permissive one, but that places us in the position of having to make those evaluations. As soon as a community has, say, CC and GPL both in play, that gets harder. We don't want to be the arbiters of legally-binding matters; we want to get out of the way and allow authors and communities to control their content licenses.
When you use other people's work in your posts here, you should follow our quoting and attribution guidelines regardless of where the content came from -- SE, Wikipedia, somebody's blog, or wherever else.