So every old map has Tartaria depicted with borders and so on, just like any other country and every old history book or dictionary or encyclopedia describes it as a country. Why do historians call it a geograpical region? What's their evidence for this claim?
Here is an answer I gave on the topic.
The long and short is that a lot of encyclopedic and cartographical works, especially in the 18th century, when describing a "Tartary" Empire, are mostly using that to refer to "the former lands of the Mongol Empire".
There definitely was not a "Tartaria" as a full fledged empire in that period. We know so, because the area was ruled by the Russian Empire, Qing Dynasty, local states like Khiva, Bukhara and Kokand, and the Dzhungar Empire, which actually was a steppe-based empire in the region in the 17th and 18th centuries, encompassing much of Inner Asia until being annihilated in the basically genocidal campaigns of the Qianlong Emperor.