That is completely wrong. It's an outdated theory. There is some contribution from these ancient groups, but also a lot of future geneflow from other groups. For example,
Peloponnese has been one of the cradles of the Classical European civilization and an important contributor to the ancient European history. It has also been the subject of a controversy about the ancestry of its population. In a theory hotly debated by scholars for over 170 years, the German...
www.nature.com
If Peloponnesean people have up to 14.4% Slavic ancestry, one can only imagine how much more the people from mainland Greece would have. According to some estimates I have seen it can be up to 30%. The Greeks of Peloponnese and Gheg Albanians have the least Slavic ancestry in the Balkans.
Also apart from Slavic ancestry, there has been geneflow from proto-Greek IE speakers and Anatolia into Greek people. How do you explain the transition from Minoan and Mycenean languages to Greek without proto-Greek invasion? Are you suggesting the IE speakers just imposed the language and left?