That pergola was built a few years ago by Jason Roberson of Charlottesville, VA. I'd write to him and ask how it is holding up and what he might do differently.
One of the photos shows that the top joint was pinned laterally. See:
Note that the exifdata in most of the images date the photos as 2013, and don't show the lateral pin. But the photo I cropped and show above is from 2015 per exif data.
The slots in the bottom of the posts look like mortises for 1/2" steel plates sunk into a cement footing.
You can see high res versions of his photos by starting the slide show, then right-click -> View Image (firefox), then edit the image url to delete the questionmark and subsequent characters. For example: