I'll try to make it even clearer.  "Outer Mongolia" can refer to the following:
The northern part of traditional Mongolia during the period of Manchu control (1691-1911)
The Bogdo Khanate (1912-24) including the period of Chinese control (1919-21)
A common designation for the state of Mongolia (1924-Present)
Mongolia as claimed by the Republic of China (1950s-Present)
An English metaphor for a generically remote place
All of these meanings should be covered in the article.  You don't have to agree as to whether the usage is "correct" or not; it exists and is attestable. —  AjaxSmack