First of, AFAI understand relative fat oxidation depends on your power zones not absolute power. The plots you posted give absolute fat oxidation in terms of absolute power, and without knowing the athletes’ zones, they don’t provide any evidence one way or the other.
Do you have evidence that strong athletes have a higher relative fat oxidation rate compared to more average athletes? That is, is the percentage of fat oxidation relative to total energy expenditure markedly different?
The topic of fueling Z2 workouts has been discussed numerous times, there is a wealth of information in old threads. One relevant study was reference by @sryke in this thread: researchers compared fat and carb oxidation rates among subjects that were given carbs and a group that was given a placebo (i. e. the drink was sweetened, but contained no energy). You see that fat utilization rates are broadly comparable, especially for the first two hours. The graphs begin to diverge when muscle glycogen is getting depleted. But you will also notice that the placebo group stopped after 3 hours, whereas the carb group kept going for another hour.
You could point towards the very end of the endurance ride and claim that there is a 12–15 % delta towards the tail end of the 3-hour ride the placebo group had to do. I reckon the reason is that the bodies of the placebo group athletes simply had no choice as their muscle glycogen was getting close to being depleted.
But I would still think that riding another hour not only burns more fat in total, but has a net positive training benefit.
There are also plenty of threads on endurance sports and a keto diet or on becoming fat-adapted. Others are focussed on ultra events. In most cases the reasons against fueling with carbs has nothing to do with performance benefit but e. g. the athlete having type 2 diabetes or being unable to carry literally kilos of sugar for several days in unassisted super long-distance races.
You used the word “spike” in your previous post when you talked about blood glucose levels. You are right that you did not explicitly mention this. But typically, the word spike is used in the context of maladaptations.
Even if you did not mean to suggest that, I still think it is relevant to mention that changes in blood glucose levels during exercise are to be judged differently than when you are at rest.
This is not about your or my choices, you hypothesized that above average athletes are faster, because they are able to burn more fat. In absolute terms that is correct, but then so is the statement that stronger athletes burn more carbs (in absolute terms), too.
If there were a consistent performance benefit to becoming fat adapted, the pro peloton would have adopted it yesterday. (Ditto for e. g. a vegan diet.(*))
(*) I’m only claiming that there is no convincing evidence that a vegan diet improves performance, but I’m not claiming it hurts performance either. AFAIK if done right, you can go vegan on the pro level without any impact on performance — positive or negative.