Somehow over time lemonade became to mean the same as soda here in Belgium. Probably through being introduced to American lemonade and Soda at the same time.
Most carbonated fruit drinks are called lemonade. Coka Cola is called just Cola. Sprite, Fanta and similar drinks are all lemonades here.
That idiom only exists in media from the USA. I heard that idiom for the first time in Portal2.