It’s difficult to parse through the long off topic debate in here, but I just wanted to interject and say that Fatmax is certainly actionable and in fact is a very important consideration for the guys doing what is essentially pro level volume.
In short the TSS of a ride will be much lower for the same level of effective training of z2 focused systems by staying on the correct side of fatmax.
The power v fatmax curve is roughly a parabola with the same effective fat oxidation rate on both sides of the apex, however as I said above the lower power side of that apex will result in less TSS per ride which carried over to an increased ability to train during that block (possibly in higher zones as part of key workouts)
But knowing exactly where that is for an amateur is not super relevant as they are likely not pushing it in terms of the hours on the bike or maxing out on TSS for a block. They can afford to be 10w below fatmax just to be safe.