Even if your premise is right, your timing has to be right too. The SP500 has doubled since 2014 which lines up with the traditional 7% per year. Meanwhile they're paying 12% per year to short, so their approach is a compounded 22% worse[1] than VTSAX-and-chill (before even considering capital risk). This is still gambling, just not in the usual direction.
[1]: 1.07/(1-0.12) = 1.22