How the hell he did it ?

Hidding PBN links is one possible aspect. However, it may also depend upon the keyword search volume and competition for which this website is ranking.

This website used to get 7K traffic in Sept. 2015, but now it receives only 500 visits and 1% bounce rate (rough estimate from similarweb). PR doesn't matter much as there are many other factors involved right now.

Greek websites may be there is less competition and seems they have a good play on social media by sharing.So just chill PR can not make your site on TOP.