It is not all that clear on this phone-picture, but not less true. The mausoleum is gone.
The discussion on the why's and the how's is still going on in the UB Post
Choibalsan was most likely killed in Moscow. As most leaders during socialism in Mongolia - including Sukhbaatar, Genden and many more - he was sacrifeced for the state. Traditionally Mongolians don't burry their deaths, but leave them on a significant place to be reunited with nature. But Choibalsan recieved an offical state funeral. A film made of this event was later shown all around the country to teach people how they should deal with their deaths.