just because a certain people invade another doesn't mean they wipe out the native population or even change it much (although sometime they do). Rome for example left almost no genetic legacy in Britain despite ruling it for centuries
Also at least some of the Sea Peoples like the Achaeans/Ekwesh were themselves a Greek people
Re-posting part of reply to address this
Also at least some of the Sea Peoples like the Achaeans/Ekwesh were themselves a Greek people