""Purchases made using DN points (DNP) are NOT eligible for this event.""
this really made me sad..
Being a business, they want you to top up real money to spend in their game. Hence if you pay cc to buy DNP vouchers, that is still considered as investing money to the game. Buying DNP vouchers is counted.
However, if you PAY DNP to the cash shop, they won't count it in. Because there's no actual act of topping up "investing" in the game.