Yaarel
🇮🇱 🇺🇦 He-Mage
To me the flavor matters, in some ways more than the mechanics. I value mechanics and flavor equally.I agree that cosmology belongs as DMG suggestions, but I've never had an issue using the Cleric class for non-theistic or non-religious characters. As long as the player and DM are on the same page flavor-wise, the actual mechanics are fairly agnostic with regard to power source.
I need the flavor of the Cleric class description to welcome nontheistic religious traditions as well as theistic ones. This also relates to reallife inclusivity. Not all players come from theistic cultures, nor are especially interested in theism.
The Cleric class works better when more inclusive. This likewise helps the class be more versatile for different kinds of D&D settings.
Setting including cosmology belongs in the DMs Guide. Not the Cleric class.