The link you provided took me to some document which is in Greek. There is a picture there, but i cannot verify that it belongs to UNPO. Then, the most updated information is usually taken as the actual information. Or should we stick to infos from centuries ago? CIA is not the world official source of information. I have seen that sometimes it has mistakes and erroneous  information on some countries. Anyway, i checked it and...Surprise, this is what i saw:
Ethnic groups:
TABField info displayed for all countries in alpha order.
Albanian 95%, Greek 3%, other 2% (Vlach, Roma (Gypsy), Serb, Macedonian, Bulgarian) (1989 est.)
note: in 1989, other estimates of the Greek population ranged from 1% (official Albanian statistics) to 12% (from a Greek organization).
So, the percentage is 3, and not 6 like you claim. There is no source where it says that it is 6%, thus it has to be removed. And bringing the claim of a Greek NGO is not reliable. I can bring you tens of claims of Albanian and other NGO which claim that Greek population in Albania is smaller than 1%, or that ethnic Albanian population in Greece is higher than 15%. (  )