In the late 1800's/early 1900's there were actually more white people in Indian Territory than there were Indians.
When Indians Lands were opened up in the Land Runs, the region was flooded with white settlers.
So, no, the fact that someone was residing in IT does not necessarily mean they were Indian.
I found the family in the US Federal Census.
The 1880 census is free online.http://www.familysearch.org