I believe the engine should be operating at varying speeds during the "break-in" period, thus the dealer's advice of don't use cruise control
From the 7th Edition Owner's Manual -
ENGINE BREAK-IN RECOMMENDATIONS
A long break-in period is not required for the engine and drive-train (transmission and axle) in your vehicle.
Drive moderately during the first 300 miles (500 km). After the initial 60 miles (100 km), speeds up to 50 or 55 mph (80 or 90 km/h) are desirable.
While cruising, brief full-throttle acceleration within the limits of local traffic laws contributes to a good break-in. Wide-open throttle acceleration in low gear can be detrimental and should be avoided.
The engine oil installed in the engine at the factory is a high-quality energy conserving type lubricant. Oil changes should be consistent with anticipated climate conditions under which vehicle operations will occur. For the recommended viscosity and quality grades, refer to “Dealer Service” in “Servicing And Maintenance”.