Both fact, and fiction.
There IS a zone of activity that does burn primarily fat for fuel. That’s the good news.
The BAD news is, you can burn MORE TOTAL CALORIES per UNIT TIME by exceeding that zone.
Since most of us have a limited budget for time, and since calories in vs calories out is 80%+ of the weight loss equation, it makes more sense to go for caloric burn in total and ignore the « fat burning zone », because if you’re in a calorie deficit, at the end of the day you will lose fat.
How much time you got for cardio? Fill it. Fill it with exactly as intense an amount of cardio as you can possible do, at a steady state, for that time, and that’s the most calories you can burn and the most benefit you’ll get from it for weight loss.
Pro tip, diet is 80% of losing weight. If you’re not counting calories and weighing and measuring portions, you may never have success with it.