What I'm about to type may have absolutely zero relevance to the Malibu ECM programming, but it's something worth considering.
Back when the emissions stuff was just getting started good. Chrysler did something called "LeanBurn" (and GM did also, but I forget what they called it - the '82 Skylark we drove for a while with the V6 did this also!) where it would keep leaning out the motor and advancing the spark until it started to ping or knock, then it would enrich the mix and retard the spark.
With that, the higher the octane, the better the gas mileage.
My 2000 has a knock sensor - don't know what all it does for the car in programming the ECM, however, so take this with a LARGE bag of salt
But that would explain how a higher octane gas gives better gas mileage. Since the octane has NO effect on gas mileage past the knock resistance - and with higher octane, you can run leaner and with more spark advance, both of which help gas mileage.
It'd be nice if someone who knows how the ECM is programmed (i.e., a tuner) could say if I'm right, or if I'm just blowing smoke up here ...
Again, I have NO idea if this is applicable to the current Malibus or not. This used to be done by GM and Chrysler, though, so ...