It's debatable. There are 'trade secrets' in all the chili parlors chili. Skyline I have heard uses unsweetened chocolate (if so). A lot of copycat recipes will also include it.
If you're much for books and history, I highly recommend Dann Woellert's The Authentic History of Cincinnati Chili. It gives a lot of background on the dish: https://www.amazon.com/Authentic-History-Cincinnati-American-Palate/dp/1609499921?ref=d6k_applink_bb_dls&dplnkId=33d4a330-dc13-4e49-9743-fcc25e33a508
This book is a great read and I highly recommend it. It really goes into depth about the history of it.