How is the title of "National treasure" attained?
I've yet to read this but I think I found the answer. Hinomaru is someone who has the title 'National Treasure' because he


  has the potential to become a yokozuna. (Wiki)


Yokozuna is the highest rank in sumo. 'National Treasure' is more like a moniker or an informal title rather than a formal one that can be obtained. It is not an official rank but a nickname and since only those with a potential to attain the highest rank are given this nickname, then whether or not he ghosted, he still has the potential to become yokozuna, hence he was able to keep the 'title'.