What is the difference between fruit teas and pure fruit teas?
Teas made with fruit and herbs, or fruit and tea, are often referred to as fruit teas. Teas made with only fruit would then have to be called pure. However a quick look online suggests that "pure fruit teas" also includes some of the former.

The article you link seems to use pure to mean "not containing tea*, as it discusses caffeine quite a lot. The sugar aspect is probably a distraction as even a fruit tea made by soaking dried fruit in hot water can only deliver very little sugar as the total weight of the dry tea is small and most of that is discarded.