A nitpick about the title: Not strictly abdominal fat, but visceral abdominal fat, which surrounds the organs. Not all abdominal fat is visceral; in fact, in many people the majority is not. The article mentions visceral early on, which is the subject.
If you have a lot of abdominal fat then you also have visceral fat so it's just a proxy
This is not true.
It is what is called “skinny fat“.
If you are skinny fat you still have noticeable abdominal fat. You might not see it when somebody has a t-shirt on but it's still very obvious
To put this another way skinny fat just means you are not overweight but you have a high body fat percentage (eg lacking lean mass). So while you might not look like the average American there will still be significant visible abdominal fat, likely love handles, etc
It's not a remotely good proxy
Seriously, you don't think there's a connection between visceral fat and abdominal fat even though there's plenty of evidence that there is along with just common sense?
It's just a particular type of abdominal fat so obviously the more abdominal fat that you can see it means you also have more of the type that can't see as well... It's not that complicated