The problem is that physiology isn't so simple, at least not as simple as you seem to believe. That is, what you say is indeed true, *but only when a very small fraction of the total muscle mass is contracting, such that maximal cardiac output is not limiting!* In this situation, blood flow per unit muscle is much higher than it is during whole-body exercise, such as running or cycling, and the fractional extraction of O2 by muscle is correspondingly lower. The peak rate of O2 uptake *by that muscle* is therefore determined by local factors, e.g., capillarization, mitochondria.