You could argue that high sugar diets are just a proxy for obesity, so if obesity has been ruled out as a cause for this new increase then high sugar diets can be ruled out as well. Plus, no real reason it would be confined to young white people only.

But I still avoid the stuff, because it seems so unlikely that the quantity of sugar we consume is just harmless.

I dont think you can make that assimption at all. A lot of is have conditions or habits that result in modifying our eating habits and movement habits enough to always be thin even with high carb diets.