That doesn’t make any sense to me. After you hit ketosis, you are using fat as a primary fuel and thus you are burning approximately your BMR + active kcal of fat each day. Fasting for longer won’t increase the calories you spend (matter of fact, it might decrease since some people feel sluggish, cold, etc).
After your liver is empty of glycogen, you will be burning all fat. This takes 48hrs for women, and 72 hrs for men. It’s a mix before hand. My ketones usually starts near 0.0 and head pretty linearly to 1.3 (at 72 hrs) and then bam 2.3 or above right after.
So, very roughly, after 3 days you are burning near all fat.