The minimum power is what is needed to fly to the closest diversion airport. This could be at a lower altitude and reduced speed. It is substantially less than

The minimum power is what is needed to fly to the closest diversion airport. This could be at a lower altitude and reduced speed. It is substantially less than the power used in take off or go around. Airplanes work in a variety of conditions and do not need 100% to get home in an etops situation. The rule you talk about is more clearly applicable to rockets that run at 100% for most of the flight.