[corresation]

One of five possible definitions includes losing weight in no universe means that diet means "lose weight" -- you don't get to choose what Diet means on a can of soda.

Diet is nothing more than the things you eat, and a "diet" can be to lose, gain, maintain weight, or simply to pay attention to what you eat for the purposes of healthy eating. A diet soda is a soda modified to reduce/eliminate sugar, which again is in no way manifestly correlated with losing weight.