I will counter-pushback. I don't think I have encountered a single person in the past 15+ years who denied or was unaware of the basic thermodynamics of "calories in, calories out". What I do see time and time again is people with manageable appetites pushing "just eat less" with keyword "just", acting as though people's failure to do so secretly reveals a preference to be overweight.

People who do keto, paleo, eliminating sugar, vegan or whatnot do this as a strategy for eating less, through a variety of mechanisms, generally some combination of attempting to stabilize blood sugar to forestay cravings, eating fewer meals, or eating less calorie dense foods (you have to eat a lot of vegetables to get fat!)