You are mistakenly thinking of FTP as being defined by a value on a mean maximal power duration power curve.

FTP is perhaps better thought of as a property of the curve, than being a specific point along it. It's a functional expression of underlying sustainable aerobic capability expressed in wattage terms. As such it's conceptual but a very useful concept because of the utility of power measurement.

Think of the MMP curve as being a function of a few (useful) conceptual things, with FTP being one of them, i.e.
MMP(t) = f(FTP, A, B), where A and B represent other useful functional expressions of underlying physiological capability that are sufficiently independent of FTP and each other.

Since the mean maximal power curve either side of about an hour is pretty flat, then seeking precision in terms of duration is not required in any case when using this method (i.e. maximal efforts of about an hour) as a means to estimate FTP. IOW don't confuse means to estimate/establish FTP with a definition of FTP.

Naturally, the further you move away from longer durations of about an hour and towards shorter ones, then then MMP expression becomes less dominated by the FTP term (i.e. the A and B terms start to play a greater role in defining the shape of the curve), although FTP is still a dominant factor down to short durations of a few minutes, which should not be surprising given the nature of energy contribution of the various metabolic pathways to maximal exercise over various durations.

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