I call it nutrition instead of diet
And you'd be wrong because they are different. Diet is diet. What you regularly eat and drink is diet, period.
When someone says diet, it doesn't necessarily imply a specific special intake of nitrition. It's all diet, even if you only consume twinkies, or eat regularly and nothing specific. It's a diet no matter what.
When someone says "I am going on a diet" which is more or less what was being discussed it does mean a particular plan of intake. It also frequently means a short term, unsustainable plan.
It is helpful to think of nutritional intake changes outside the context of "going on a diet" because setting long term, sustainable changes is going to work out better.