It is wrong.
Yes, the engine may be at it's most efficient power/fuel point, but to say that the engine as air pump burns less fuel at 4000 rpm when at 2000 it is using roughly half as much air and therefore fuel is just wrong.
The term roughly is due to the volumetric efficiency reaches a maximum at some point - say for a N/A DOHC 92% @ 4000 and only 88% @2000, but if there is less air our modern FI will portion to fuel accordingly.
Better percentage at torque peak - less fuel at lower rpm.
El Sid
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