I don't have ADHD, but I feel like this is perhaps too simple of an answer.
I imagine, when the first diagnoses came in, people were not aware that they experienced life differently than others. They probably assumed that everyone else struggled just as much, but somehow was better at it. At least, that still seems to be a common theme when kids get diagnosed today.
And so, of course, past kids didn't go to the doctor for what they assumed to be normal, but rather got sent there by teachers who described what they saw from the outside.
I guess, I mainly take issue with this post implying that Big They™ intentionally chose a bad name, when there's a fairly simple explanation why this would happen without a conspiracy.