Why there are similarities between Greek and Hindu mythology?
The proof is in the pudding, as the expression goes. However, your question is on Vedic & Greek mythology (about gods) whereas in your description you ask about Aryan migration (about people). These are two different questions.
From reading the Adi Parva of Vyasa Mahabharata (or from Bala Kanda or Aranya Kanda of Valmiki Ramayana), we come to know that the Creator created Prajapatis first and asked them to procreate and fill the planet with life. Sanskrit 'Prajaapati' means 'Lord of people' in English. Then, Vedic theology is all about the descendants of 'Kashyapa' Prajapati, son of Maareechi Prajapati, who is son of Creator. Kashyapa prajapati had 28 wives that we know of. Most important of his children were called 'Aadityaas' (12 in number), 'Rudraas' (11 in number), Vasus (8 in number) and Ashwins (2 in number). These 33 gods and all their descendants are collectively referred to as the '330,000' gods or '33,000,000' gods or '3,3000,000' gods in Hinduism. These great grandsons (sons of Creator's grandson, Kashyapa) and their step brothers (brother from another mother) and their cousins are all collectively called in puranas as 33 crore devas. 33 crore and not 31 crore or 45 crore because these 33 were the most important ones among all of them, most important to vedic peoples actually. And crore because no one could count all of them (as you can imagine they wouldn't be in living in 1 spot and were dispersed making humans counting them impossible).
The most important of the 'Adityaas' was Indra, and he was crowned as King of gods some where in modern Armenia region and his kingdom existed somewhere in modern Siberia and Northern America (per Kishkanda Kaanda - SUgreeva's instructions to his commanders in Vaalmiki Ramayana). The other most important 'Adityas' were Vayu (wind god), Varuna (god of justice and waters and oceans), Agni (god of fire and volcanos and heat and the primordial Sun).
Per Homer's Illiad and Hesoid's poems we learn that their gods descended from Uranus and Gaea (pronounced Geya). The pantheon of ancient Greek gods starts with their children. Uranus or Ouranos is believed to be the 'Varuna' of the Adityas by mythologists and historians, i.e. Varuna was called as 'Uraanus' and 'Ouraanus' in the west, and per vedas Varuna is the lord of the western direction. The ancient Greek pantheon begins with Uranus/Varuna and his wife Gaea, and their children (called Titans) who were later supplanted by Titans' children, called Olympians.
So imho, our 33 vedic gods were great-uncles of Greek Olympians, i.e. their were the grandchildren of one of the 33 primary vedic gods Varuna. Naturally, they ruled and lived in those western lands, whereas the 33 vedic gods ruled other lands, Indra ruled east, Agni ruled south east, Vayu ruled north west, etc.
This is also the reason why all Vedic concepts are found in the greek mythology (cremations & not burials, animal sacrifices, linga worship as 'Omphallus' stones, etc.), but all greek concepts are not found in Vedic mythology (ex: ancient greeks' obsession with anal sex, sons killing fathers is not a sin, ancient greeks' obsession with maternal incest, no importance given to businessmen and vocational folk, etc.). Obviously, if vedic gods are plagiarized from Greek ones, we would have imported all these missing greek concepts also. But they are missing in our ancient religion. That is because there was no plagiarizing between ancient vedic and ancient greek religions. The author of your article is incorrect.
However, subsequent to Alexander's loss to Porus and his vengeful return back to Persia, several historians hypothesize that several vedic concepts such as vedic astronomy, concepts of vegetarianism (look up Pythagoros), etc. were imported by the greeks.