
You're 100% correct; but it wasn't just GM.
Every OEM got crappy mileage compared to today. There are many reasons for it; one of the big one's was how to meet emissions. You have to remember that these older vehicles, if they had computerized engine control at all, might have had an electronically adjusted
carburator. That's right, carburetor.
Before EFI, with it's high speed data feedback capabilities to keep an eye on what was going on inside the combustion chamber, one of the easiest ways to control emissions was to keep combustion temp's down. One of the easiest ways to do that was to run the engine on the rich side. There's a whole lot more to it, and I know that I'm probably never going to understand more that the general outline of the topic.