Necropolitan
Hero
Even if that wasn't the case Tiamat stepping in to take control of the Cult of the Dragon made complete sense.I’ll have to see if I can find my original posts to confirm, but IIRC the idea that Sammaster had misinterpreted the prophecy dates from the 2e Cult of the Dragon book, and the 3.5e Dragons of Faerûn book established that Tiamat’s cultists were infiltrating Cult of the Dragon sects. The way the Cult of the Dragon was presented in ToD was a natural evolution of that original lore.
With their greatest achievement (The Dracorage/Rage of the Dragons) undone and thus their mistranslated version of the prophecy seemingly thwarted and their founder/leader dead the Cult could easily be taken over by a dragon goddess, especially since Tiamat has her Undying Queen aspect (a Dracolich version of her normal form) which would definitely appeal to them.