Yeah, GDP really measures size rather than quality, people just end up ignoring the nuance.
GDP is also significant in that it directly influences how much revenue a government can raise by taxes, and so by proxy how much of public services it can provide. GDP growth also influences what amount of deficit it can sustain (though for various reasons that is not a straightforward relationship as with tax revenue).
Whether you burn fat or carbs doesn't matter here, if it's the same amount of calories.
There are credible arguments you can make for why eating specific foods would help weight loss, but this is not one of them.