Interesting takes on this. The comfortable solo travelers tend to see many other solo travelers. But I would surmise this is because they avoid places where the massed American contingent go (Times Square for example)
The American Plan travel system does constitute the bulk beyond US shores. Its main function is to allow its tour patrons to see and take pictures all while in the familiar bubble of food, beverage, & accommodations. Not to mention never needing to deal with all those who refuse to speak "American"
Of course that has been my observation over decades.