How does buying more of a foreign currency devalue a nation's currency?
Simply because buying country A's currency priced in country B's currency means selling country B's currency.

Simplified but not simplistic example: say that you have 80¥ and want to buy 10€. When buying/demanding these 10€, you will actually simultaneoualy also sell 80¥ i.e. by doing so you become a supplier of (€-priced) yuans. And because selling an asset generally exerts a downward pressure on its price, the price of 1¥ expressed in euros will decrease. Hence the explanation you read.