The grant is indeed confined to students with an EFC below 15,000; this is below the total Cost of Attendance at UNC so these students would be eligible for State and federal aid at UNC.
You are mistaken about how the "need-based" grant to students at private colleges in North Carolina actually works. In fact, every student who receives these grants would be eligible for need based aid at any UNC campus as well. It is not related to the total cost of the private college but instead is related solely to the calculated ability of the student and family to pay those costs - exactly as it is calculated by the federal government for UNC students.