As best as I can understand it, which is limited, is that he needs to be certified to be on track to graduate during the semester by UNC. That is a separate "eligibility" question than the NCAA and admission. Every athlete is certified by UNC during the season as being on track, we just never hear about it because it is a formality in most cases.
With Luka, we have apparently put him in a certain academic situation that may or may not be correct based on his prior classes. Depending upon whether those classes count or not could apparently impact whether he is on the correct path right now or not, and hence whether he is eligible.
What I don't understand is why there couldn't be some sort of "we will figure this out later" ruling given the unique situation Luka is in. Our compliance department has been poor -- the Tez Walker paperwork situation being the worst example of this -- and I suspect this may be an example of someone requiring boxes to be checked.
My best guess is that the box checker will be overruled if the Montenegro paperwork never arrives and that we will just make some exceptions for this case. But it is UNC so who knows.