15 minutes really does work, but it has to be intense - like, actually intense (it's not the same as a casual jog versus doing burpees full out). The thing is, those videos aren't exactly lying, they just leave out the part that those 15 minutes need to be at almost your max capacity, and it also depends on what you've done before in your life - if you've never trained anything, it's going to help, but if you already had an hour routine, dropping it to 15 minutes is going to be less effective. The real key is consistency: 15 minutes every single day is infinitely better than an hour twice a week when you finally find the time, and if your reality for the next 3 months is that you can't make it to the gym, it's better to start with that than do nothing at all.