tricky one without duplicating the advert.
That site is actually inserting it into the main content and on larger devices its absolutely positioning it to the side.
1. Create the narrower content by having left and right sidebars displayed.You can leave them empty by adding a HTML widget with just a HTML <!-- comment --> inside of that.
2. Place the advert inside the post content. And whatever container its in give it a class so we can target it.
If you have a test site like that set up on we could take a look.