Having grown up in that area of the Midwest, I largely agree with the author's categorization, except that "people on their way to somewhere better who got tired and decided this was good enough" describes a LOT of the midwest, not just Indiana. Significant chunks of Michigan, Ohio, and Missouri, most of Iowa/Kansas/Nebraska, etc.
If you read the history of westward expansion, "got tired and decided this was good enough" is literally true for how much of the area got initially settled (by white people)
And even before Gary Cooper there were people using it for Gerald (spear power), Gerard (spear hard/brave), and (old) Gerbert (spear bright). It is a cousin to, but believed not historically derived from, Garrett/Garrod. It is unclear whether German/Germain derive from this root or not. It is usually unrelated to Jared (which is usually a Hebrew name, but does have spelling variants that overlap Garrod).