When I was going into MIT App Inventor, I made a typo and went to https://ar.appinventor.mit.edu/ .
I logined by my google id in that site. The site was almost same as MIT App Inventor. But the difference is that there is something called Augmented Reality in the Palette and it is an older version of MIT App Inventor. If you can't belive it go to the site: https://ar.appinventor.mit.edu/
With regard to the ar server, this is a test server created by one of the MIT students for development work. It is not an official server, otherwise it would be listed as one. I suggest you do not use it, for anything.
It was originally used for a student's thesis and uses an iOS companion that was never even published. When you use the site you are warned to backup your projects and use them somewhere else.