Genetic differences can exist and not be meaningful if the situation doesn't allow for them to be meaningfully expressed.

We know that people just have different metabolisms, different levels of hunger response, different levels of "food noise," etc.

We see obesity rates raising in nearly every country across the world as economic power and access to ultra-palatable hyper-calorie dense food increases. This is universal across the developed world with very few exemptions - Japan seems to largely be immune, but Korea and other portions of Asia aren't.

The growth curves look very similar, despite many of these cultures across the world putting even more emphasis on discipline and responsibility than America, things that in theory directly align with being more responsible about diet. If genetics weren't involved and it was all willpower (and where does the capacity for willpower come from? Is there no genetic component to it?) we wouldn't see these growth trends be so similar to America and each other.