If you want to be technical, any habitual pattern of eating is a diet. But technicalities don't define words, usage and context do, and, for the vast majority

If you want to be technical, any habitual pattern of eating is a diet. But technicalities don't define words, usage and context do, and, for the vast majority of people, including people here, diet means a change in your habitual pattern of eating designed to induce weight loss. That's how it's universally understood in this context. Being more technical does not mean being more correct.