Chocolate is expensive, so I'd imagine they left it out decades ago. They can use other ingredients to simulate whatever flavor it might have conferred at low levels.
Obviously, various recipes call for it, I have no clue what might have been used originally. It's interesting how this "chili" took over one small region to such an extent and never spread elsewhere to any extent.
I get a bit of a hankering for it once a month, but I lie down and it passes. The other Cincinnati "tradition" of course is "sliders", White Castle, emulated down here by another chain (that isn't doing do well, I can't even recall the name now).
Then there is Waffle House which did originate down here and has spread rather widely and attracted copycats also.