I think you are misunderstanding the text. Here is the quote from the DMG:
Bahamut
(ba-HA-mutt) is one of the
primordial dragons who (along with
Tiamat) is said to have created the
First World. For
practical purposes, he is a god—ageless and immortal—who has dwelled in
Mount Celestia (see
chapter 6) since the destruction of the First World.
You don't say for "practical purposes, he is a god..." if he is explicitly a god. You would just say he is a god. This is the same language the use in Fizban's were they are even more explicit that they are not gods:
Bahamut and Tiamat, the
primordial dragons and the purported creators of the First World, are the closest things to gods among dragonkind. Since they share the same fundamental connection to the Material Plane as their dragon offspring, Bahamut and Tiamat are ontologically distinct from the gods that hail from the Outer Planes. But for
practical purposes, they are divine—worshiped by mortal creatures,