And did you actually clicked that button? Because I did and it took me a bit down on the page to the "buy a coffee" section with a default three Euros price. Later I checked it again and realised there is a "No thanks" option which actually allows you to get it without paying. But that is not the default, no matter what is displayed there.
The "No thanks" option has caused a lot of people to misunderstand.
I already "bought" this game when it first came out. So I've already forgotten how make a free purchase.