Silly question

[Mirhd]

Au Ra being somewhat draconic is plausible. The auri race share some traits with dragons, but the horns could be an independant genetic change. Descended from and being 100% draconic are separate. For reasons why they are draconic:

-they have a minor version of the adaptation trait dragons have when growing up (at certain stages of their lives, dragons in ff14 change shape and size depending on their life and nearby crystals)
Sui no sato and a tribe in the steppe can explicitly survive underwater far longer than is normal
A northern glacier tribe, the Aogurs have a unique skin tone explicitly caused by the nearby glaciers and the refracted light from them
the quiil tribe also communicate through song and melody for similar reasons dragons do- to attach emotion to meaning

-On the first, Au ra are known as Drahn. Drahn means dragon in dragonspeak
-There are the falak or serpent like dragons at the bottom of the tempest on the first. There's the hydra dragons in the world of darkness which is the 13th.
-One of the gwibers explicitly states it was an attempt to recreate dragons. Given Vrtra confirms that the shards had been split before Middy arrived on the source, this means at least some dragons found their way to the first before the flood of light.
-Ratatoskr was "A wanderer and bearer of dragon's songs" and has been implied to have gone to the furthest reaches of the skies to bring messages between her kin. Given the above its not unfounded that she found a way to the other shards at some point. Nearly all au ra are nomadic or at least have a sense of wanderlust. Dragon broods do take after their lineage fairly strongly and unlike other migratory tribes that are slowly stopping like the miqote, the auri are still going.
-Au ra have no set age or maturity rate (the statement about races not mentioned living like hyurs was made before the auri race) So do dragons

Pretty much everything you stated is often why people mistake them for having draconic ancestry in the first place. Still doesn't mean Au Ra (or anything with similar traits) = dragons. Au Ra existed on the Source before Middy ever showed up and official sources (doesn't get more official than the game itself) are pretty clear that they have nothing to do with each other. The game also makes it quite clear that dragons are from another planet; hell, we visit a partial memory of that planet in Endwalker. For Au Ra to be descended from dragons the devs would need to throw lore out the window.

EDIT: I don't recall that group of Au Ra in the Ruby Sea being stated anywhere to have some super natural ability to breathe under water...? What does that even have to do with dragons, given that's not something associated with them, really? I could be forgetting something (insert insomnia complaint here), but even discounting that, we, the player, gain the ability to breath under water via a blessing from the Kojin, so it's not unheard of. Also, the "Drahn" thing is very likely meant as a joke, not some definitive confirmation.

EDIT again, don't post when low on sleep, you fool!: I doubt any of Middys' kids can traverse the Rift (space) as he can, probably relating to his extreme age and how the older the dragon= the stronger they get. The game goes out of its way to remind you how powerful he is (was) yet tells you traveling through space taxed him severely, and he's never quite been the same. Not enough time to recover, I suppose. Each time we've seen/heard of him, it's about him waking after a long sleep to do some stuff (fight off the Garleans at Silvertear, for example), then immediately return to sleep to restore himself... none of his children seem capable of the same feat (Middy has "died" how many times, now?), so again, I doubt they can travel through space.