Blood of Dragons (spoilers RotE incl City of Dragons)

['thul]

oh? it does not confirm it. Salt, while descended from Elderlings, may easily have been more six duchies than Elderling. It was remarked clearly that the dragon statues carved by Elderlings were inhuman in their thoughts. Over the generations since the breaking of the world and fall of dragonkind, there have been hundreds, if not thousands of skilled ones departing to carve dragons, long after the connection to dragons was forgotten.
The fact that some Elderling was carried in a basket looks to 'thul like proof that not even Elderlings were allowed to sit astride a dragon. Besides, you'd either need a very strange saddle, of which convincing a dragon to wear would be very difficult, if not impossible, or extremely tough skin on your thighs to last long from the friction of scales.

The book cover you mentioned does not say that Elderlings used to ride dragons, but instead open up for the chance that they (the new Elderlings) will come to ride dragons. In the ancient times, when both dragons and Elderlings were more common, their relationship was more objective. Elderlings were objects of art, whereas in the time of the books, Elderlings have become more of companions. It is clear that they have more power over dragons than Elderlings had over dragons in the old days.

Oh, and wasn't Reyn carried in a limb, not mouth? These beings clearly seem to remember (malta?) observing that in the combat scene Tintaglia was holding a single limb close to her body, and was thus less effective. Understanding why this was so was something she did not do at first.