I will post my firsthand knowledge. I'm over 50 years old now, in 2022, and grew up in Romulus, MI, during the 1970's and 1980's, right next to Metro Airport. My grandparents were hillbillies in the sense the word was being commonly used at that time. My grandmother was from Kentucky, and my grandfather was from TN (he had hoboed up from the South back in the day to work in the auto factories). They were poor, simple folk, but they were neither "white trash" nor ignorant. I had heard the word "hillbilly" all my life. Imagine my surprise when I opened the dictionary at school (don't remember which one it was, but I seem to recall it as a common large red dictionary used in classrooms) around about the fourth or fifth grade and read for myself that a "hillbilly" was a "Michigan hill farmer." Whatever the definition now and regardless of how it was commonly used during my childhood, at some time previously it had meant something somewhat different. Etymologies listed today neglect to mention that fact, but I can attest to the veracity of the claim made by the OP. My guess is that you'd have to lay hands on a dictionary from between 1960 and 1980, but that's only a guess.