I don’t think that anyone has been able to show any chemical difference between fructose in soda and fructose in fruit.
But there's a huge difference in how fast they are absorbed.
Ever been to a BBQ and had four beers over four hours? Sat at a bar and slammed four shots in four minutes? They contain about the same amount of ethanol. The effects are very different.
There's a rate at which the body can process fructose. Eating it as a solid, bound up in fiber, as in fruit, results in an extended period of moderate levels of fructose entering the system. Drinking the same amount as a liquid results in an extremely rapid, extremely high level of fructose. It's not at all inconceivable that this would make a difference.
I don't think anyone has suggested that fruit juice, for example, is as dangerous as HFCS soda, and I have never heard that carbonation, per se, is deleterious to your health.
I had a doctor tell me that I could safely eat cantaloupe but not watermelon because supposedly the watermelon had a much higher fructose content. Having grown up hauling both of them to the market by the pickup load I had an idea he was wrong. A little research confirmed my suspicions, the sugar content varies more between varieties than it does between the average of the two categories, some varieties of cantaloupe have MORE sugar than some watermelons.