I agree, there is no reason to believe hypertension is overdiagnosed. If anything it's underdiagnosed. Even without the obesity risk factor, there would still be legions of people out there with hypertension who feel literally nothing. The existence of false positives does not change that, it's an expected phenomenon in any kind of testing, diagnostic or not. It does get worse with age: pretty much all the chronic diseases become more common as people get older.
This is only going to get worse too as people who grew up overweight and not exercising get older.