The untreated persimmons that start to soften are still quite astringent until they get very soft.
I have left a few untreated persimmons on the counter and not in a container, but I haven’t made careful observations. I prefer to keep my persimmons in some kind of container because sometimes they soften up and start to leak before I notice them.
I’m sure you’re right about ethylene playing the key role in ripening, but I would expect the treated fruit in the enclosed CO2 chamber would have even more exposure to ethylene than the fruit in an open bucket, right? Unless the bananas and apples in my kitchen are producing enough ethylene to affect the open bucket of untreated persimmons down the hallway and in the bedroom 30 ft away.
I’ve noticed on my trees that any fruit that has even a small bird peck will ripen long before the others on the tree. So maybe even small damage or bruising could also accelerate ripening.
Maybe the fruit in the study was much closer to fully ripe when treated with CO2 than the fruit I pick and put in my CO2 chamber. Or like you said, maybe there are differences between cultivars. Most of my treated fruit has been Giombo and Tecumseh. I’ve also treated a few fruit off my younger trees - Smith’s Best/Giboshi, Eureka and Nikita’s Gift. Interestingly, the three hybrid Nikita’s Gift fruit I tried were still too astringent to eat after 3 days of CO2 treatment, while all the kaki fruit were non-astringent. My young Nikita’s Gift tree only had three fruit on it this year, so I can’t try any more and see if a longer treatment period would remove the astringency.