The problem with modern nutrition is that people consume too much calories, not too few.

I'm pretty sure you're referring to the poor health outcomes correlating with high caloric intake, which afaict, come from people's consumption of heating oils (seed oils): https://en.wikipedia.org/wiki/Vegetable_oil

There's bound to be mixups what talking about sugars and fats. But these fuel oils are really really bad for us. We've demonized sugars unnecessarily. Right?

No, exactly opposite. Fats are good, sugars are bad.

Good fats are good, but there are bad fats too!

Get your nutrients from everything else, you don’t need sugar for nutrients, and that’s not why you put it in a chocolate cookie