Take a couple of scenarios....
1) 70mph on the motorway, you reach an incline where the torque requirement to maintain your speed is zero. Does it use more fuel with the engine turning at 800rpm or 1600rpm?
2) You you lift off the accelerator approaching a roundabout and gently apply the brakes. The torque requirement is greater than the engine can provide through engine braking. Does it use more fuel to provide maximum negative engine torque at 1600rpm or zero torque at 800rpm?
Your explanation covers the 2nd scenario but ignores the 1st. Hence it is sometimes better and sometimes worse, scenario dependent.
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