While their nutritional quality has gone down tremendously, for vegetables too: https://pmc.ncbi.nlm.nih.gov/articles/PMC10969708/
Here is a list of approved bioengineered foods in the US:
https://www.ams.usda.gov/rules-regulations/be/bioengineered-...
All the fruits on the list are engineered for properties other than sweetness.
The term you're looking for is "bred". Fruits have been bred to be sweeter, and this has been going on a long time. Corn is bred for high protein or high sugar, but the sweet corn is not what's used for HFCS.
Personally, I think the recent evidence shows that the problem is not so much that fruit is too sweet, but that everything is made to be addictive. Satiety signals are lost or distorted, and we are left with diseases of excess consumption.
This phrase is nonsense, because HFCS is a chemical process applied to normal corn after the harvest. The corn may be a GMO but it certainly doesn't have to be.