Anecdotally, my sister's kid just got diagnosed with it. He is not overweight whatsoever, but they put frickin' ketchup on everything. But, yea the majority might be overweight, but anyone that eats just carbs could get it.
If there is some kind of direct damage occuring to hormonal systems then it doesn't really have to correlate to weight and exercise related T2.
The roads of my childhood were definitely included higher polluting diesel trucks, but kids and pregnant women did spend a lot less time traveling and usually didn't ride in diesel vehicles themselves, there was that fine leaded gas..