The level of I2a in Greece proper, aside from Greek Macedonia, is even at lower levels than in Albania. Overall, Greece has the lowest levels I2a in the Balkans. So even if it is a newly introduced strain in the Greek population, it is at very modest levels to say the least. Even Greek Macedonia has lower levels of I2a than any Slavic speaking country. Unlike other peoples in the Balkans, Greeks have inhabited various geographic localities since ancient times: Asia Minor, Alexandria, Cyprus, Black Sea, the Aegean, the Ioanian, Pelopponesse, the Greek mainland. Yet by and large they essentially share the same origin.
As for I2a. I think it has been around in the Balkans since pre-classical times. It may have been in classical Greece, especially in the north, but in lower levels compared to today. Later when the Slavs came, some of the I2a may have been displaced and went South towards Greece. After that, the Slavs (mostly R1a) migrated to the Balkans and absorbed a lot of I2a people. A portion of those Slavs which by now were mostly I2a people descended towards Greece adding some more I2a in the region, especially in the North of Greece.