can anyone explain how it comes that this site mirrors stackoverflow somehow?
This is known as scraper. Basically, they copy the content from Stack Exchange Q&A. By its nature, since every post here is licensed as CC BY-SA, it's permitted to distribute the content elsewhere, provided with required attribution.
Unfortunately, not many sites fulfill the requirement, and usually, there's nothing you can do (unless you are the owner of the post).
The site doesn't follow attribution requirements, uses a different license, or claims ownership of the content. Unfortunately, at this time, there is no action we (Stack Exchange) can take as we do not own the content ourselves, and reports regarding such content are no longer accepted or actioned. You may, however, be able to take action yourself if you are the owner of the content.
You can read the post on the main meta for more information.