general answer; (I think posters who joined some facts I ignored);
if Slavs in Greece were not of slavic blood (or DNA) how to measure their demic input? -
if the first waves of Slavs had distinctive traits inherited from their cradle, some visible % of their input would be seen even if they were not pure when reaching Greece - I'm tempted to think the Slaves which got down to Greece did it rather through central Balkans, the rivers net, and they did not pass across Dalmatia/W-Balkans; so they would have had some DNA visible input even in their journey end -
the recent Greeks show/shew physically regional differences marked enough (more concerning bones than pigmentation as a whole) and these differences seem deeply rooted in past, so even if there have been displaced pops, they did not erase at all the ancient differences in pops making -
I'm sure Greeks had Slav "blood" (at diverse levels according to regions), and as an average this is visible on PCA's (I know PCA's are not the best tools but...) -
I find some reactions on fora lack of distance to History and are inadapted to serious cool discussion.