Again.

• "taRtar" and so "taRtaria" comes from ancient Greek language and means something like "hellish". Comes from mythology as name of location. It is mythological toponym.
• "tatar" and "tataria" comes from ancient Turkic language and comes from the "tat" - "to pull", "to drag" because nomands dragged their belongings travelling. In Russian there even word "тащить" from the same origin, with same meaning - "to drag".

None of horse nomads named "taRtars" ever here.

That "taRtar" thing is purely European thing. Ancient Greek and Greek mythology was part of higher education in Europe in MidAges. So "taRtar" came into European culture along with ancient Greek literature. It was used to name something unknown and so horrible and intriguing. When some cartographer had no clue about some area on the map, he, being educated person often name that "terra incognita" as "TaRtaria". Obviously, people that should live in that fictional land named "TaRtars" by ignorant Europeans.

"TaRtaria" is exactly the same as "Atlantis", but unlike Atlantis scary and horrible. Mythical unknown land. Sometimes, due to ignorance, placed in the unknown to Europeans lands to the East of Volga river, to hide absence of real knowledge. That's it.