It may be legend, or it may be that the ECM was programmed a bit differently. From late 1985 on through at least 1994 the GM ECMs almost all had coding for Highway Fuel Mode and Highway Spark Mode operation. Even though the coding existed, it was turned off in the programming because the operation at 17:1 mixtures and 48° timing advance at light load values and low throttle angle would not allow the engines to control NOX emissions enough to meet EPA standards. Some of us who have played with the Highway Mode settings have been able to achieve more than 30 MPG highway on L98 V-8s and slightly better on LB9 V-8s in F-cars which were rated for 22 MPG.
GM pretty much had a method to increase mileage but at a cost of higher emissions of nitrides of oxygen. The result is that we burned 30% more fuel on the highway because Uncle Sam was more concerned about what it thought might be damaging the stratospheric ozone layer.