Yes, people are using LLMs for this kid of thing. Lots of people. All the time. I've met plenty of them and there loads of apps that offer this kind of "service". The authors are well aware that people are doing this and probably anticipated the result.

Why do the study at all? Because it's important to demonstrate and measure things, even obvious ones. Because it's not obvious to everyone, like the people who are already consulting LLMs for dietary information to manage their health. Because it's easier to enact official policies when there's hard evidence.