I do not believe after considering the evidence that Greeks have substantial Slavic influence. Some, but not substantial.

Instead, the difference in North European ancestry between Sicilians/Cretans and Greeks as a whole is because of other more southern-oriented ancestry in the former which diluted the northern influence that would have arrived via mainland Greeks.

Thus it makes more sense to say that Greeks have inflated North European because of their greater Indo-European influence, and less MENA influence than the islanders, not higher Slavic. And similarity to Bulgarians and the like is due to shared native Balkan ancestry.

I hope this clarifies.