Oh that, yeah, that's not new lore. Even in the Forgotten Realms, Tiamat is kind of an interloper deity herself. The thing is though, Forgotten Realms is written with interloper deities from all over the place being a part of the setting- Selune, Shar, and their "daughter" Mystra are the original deities, the rest mostly migrated from elsewhere, which is why you have Finnish deities, Norse deities, Egyptian gods, and all the rest fighting for elbow room.
Krynn is supposedly a closed system, as intended by it's creator pantheon, and it used to be that a Cleric of Pelor would be a powerless heretic there. Though I admit, cracks started forming when Spelljammer declared Krynnspace as part of a greater shared universe...