I agree this data is suspicious. How do they get at the root cause? I dunno. My uncle died last year from not eating enough bananas? 🤨
It is not just human breeding that makes fruits sweeter and less fibrous. The plant itself is under pressure to do that, to spread it's seed more effectively.
It is hard to find any fruit that hasn't been cultivated at least a little bit by humans over the last million years, even mangos and mangosteens. But some fruit has changed more than others.
Apples were created by Rome from crab apples, which are barely edible, it is not an ancient naturally-evolved fruit.
But fruits on plants where the apes live are more palatable than crab apples. Mostly figs, mangosteens, lychees, and they are not highly cultivated and bred.
I think it is a miscomprehension that fruit 10k years ago was far less sweet and far more fibrous... rather, I think our modern dietary excesses make sugar into a villian and we project that status onto all our knowledge. Yes, sugar is a villain if you are overeating, but it is nothing of the sort if you are not.
Therefore I think angiosperms made sweet palatable fruits for mammals to spread their seeds with. Also bees make ridiculously sweet honey, and we didn't breed them to do that. And hunter gatherer tribes eat large amounts of honey. Another data point against the "sugar is modern and evil" idea.