They did send their stone Behemoths after a few dragons (a Behemoth is a fortress converted for flying, think Citadel from Dragonlance). They killed Icewing's mate (Icewing = Ghostwalker) and then he got pissed and wasted one of them (the only time Thera ever lost a Behemoth). After the assasinations of two dozen key figures in Thera, all of whom profited from the assault on the dragons, Thera decided to back off. The dragons and Thera had an uneasy truce after that.
The dragons would give a Theran dying of thirst a glass of seawater, just for the chuckles. They certainly wouldn't give them knowledge of the mana cycles. The Therans figured that one out themselves, and produced the Rites of Protection and Passage to hold over the rest of the world in order to gain tribute from, and lordship over, the nations of the world. The dragons didn't like it, which was the cause of the aforementioned hostility.
It's highly doubtful that Aztlan has remnants of the Therans, given Aztlan's past history with the Horrors which Thera actively worked hard to overcome. Thera was a nation of jackasses, but they knew a bad thing when they saw it. The Atlantean Foundation is much closer to being in line with ancient Thera than Aztlan.