Yes, soda is more hydrating than diuretic, but you’ve gotta look at the practical example rather than the ear data. The amount of caffeine in soda would become a problem if you tried to stay properly hydrated by drinking soda. You’re only supposed to have ~400mg of caffeine in a day, and a can of soda has ~30-50mg on average in 355ml of liquid. 10 cans of soda might be almost enough to get the water you’re supposed to drink, but you’d be pretty much at your daily caffeine limit; any more and you’d be in danger of heart issues, doubly so because of the dehydration, not to mention all the sugar and other crap in 10+ cans of soda…

Obviously these numbers vary by person, but not so much that the caffeine content isn’t a concern for people who drink soda exclusively.