Slow running?? Since when did this become a trend?

‌I am just someone who enjoys running, weight training, and circuit training on my own, but I recently saw something on YouTube? Slow running? They say it's good for burning fat and that regular running just leads to muscle loss and is inefficient for fat breakdown??
Is this true?? No matter how good the fat breakdown efficiency is, if you do slow running for about an hour, it doesn't seem like you'd even burn 200 calories?? Is this for real?? (Of course, it seems like a good exercise for beginners or the elderly)
To lose muscle through running, you'd need a pretty significant mileage..