@Rtron
Well how would he even know about the ambush in time to interfere or adjust his actions? Ialu certainly didn't know; and Thulemiz seemed to make up the idea on the spot. Assuming that Aztoc even has scouts and his information isn't coming solely from deserters like Bjorn, I'd say he might not even find out that Ialu's lost his army of 3000 dragons.
It would be hilarious if he sold his soul to Astarte in exchange for her help with destroying a horde of dragons that no longer exists.
Or do they?