What he's talking about (the wealthy people of Ancient Greece voluntarily giving away some money for the greater good) sounds more like a charity.

I believe he said the wealther you got, the bigger burden you owe to athens, because without Athens you wouldn't have gotten rich, and we want there to be rich people in the future in athens as well.

Probably worth noting that a flat tax system would fit that description, even though it's not "progressive". I'm curious what the specifics of Athenian taxations were.