Like others said, I'm calling bull on this. I'm young and haven't had my card for a long time, but I pay it off in full each month and I'm in the high 700s. The only things I can remember reducing my score are: 1. My credit history is too young, 2. I only have 1 line of credit rn, and 3. I had high revolving utilization one month. So now I try to keep my utilization around 30-35%.
If you don't pay off in full you are now paying interest on your card. I did take econ class in high school (basically US financial education, and if it's not federally required then it's a mandatory class in my state.) and while it left much to be desired, this was one of the few things we learned. Basically the biggest lesson from that class was interest sucks and you should avoid it at all costs in anything, wether it be a credit card, payment plan, loan, etc. Always try to pay outright if you can, and reduce your credit usage to what you can actually afford. We would literally do the math to show how much more money you spend over time.