That is not true

From approx. 55% of Max VO2 power (for me approx 70% of Max HR), if you push more, fat consumption per hour is going down.

At FTP (for me approx 87% of max HR), fat consumption is nearly zero (for me less than 10g/hour, vs around 50g/h in maxfat zone).
At FTP your are pumping sugar. For me, 200g+ sugar vs 5g fat per hour. 10x less fat/hour than at maxfat zone.

Look at the curves I mention/link in my post above. These are the curves for Cam Wurf (Max VO2 power around 500w, for reference to the consumption curve). They apply for you also. You can see that after the MaxFat zone /green zone, fat consumption is going down, not only in %, but also in kcal/hour or grams/hour.

And the more you train pushing hard.... the less fat your muscle will use in the future, fat consumption going down sooner, at lower power....
This is the reason for 80/20 training.