I looked into this a bit: "Delaware County" was a short-lived county and not the same as the later Delaware County. Treaties with the Native Americans in 1818 resulted in the "Delaware New Purchase."
The area was called the Delaware New Purchase until it was divided into Wabash County in the northwest and Delaware County in the southeast on January 2, 1820. The counties were soon after dissolved, and the areas came to be called the "Wabash New Purchase" and "Delaware New Purchase" (renamed the "Adams New Purchase" in 1827). Subsequently, 35 new counties were carved out of the original area. The future state capital of Indianapolis was founded in 1822, roughly in the center of the New Purchase area.