Higher BMI correlates with various morbidities, but is not necessarily the direct cause.[1] East Asians have higher rates of type-2 diabetes than Caucasians, both in the U.S. and in East Asia, despite having lower BMIs. So, theoretically, plastics could absolutely be a factor here.

But this nonetheless bolsters your larger point, which is that we need to better appreciate how diseases manifest and evolve across different populations, with an eye toward shared but less obvious (i.e. less superficial) similarities and pathways. And studies--or at least their conclusions--which fail to do so should receive greater scrutiny than they currently do.

[1] Notably, East Asians tend to be more prone to intra-abdominal (visceral) fat, which more strongly correlates with type-2 diabetes than BMI. Plenty more East Asians are walking around with diagnosed metabolic syndrome than you would guess simply by how they look. So fat might still be directly involved, just not the obvious, subcutaneous fat which people of Caucasian and African ancestry put on faster and more evenly distributed across the body than some other populations.