I'm saying that for 99% of games, official or homebrew, that the creation of the setting is normally irrelevant. It's nothing more than setting the stage for current events, which is handled just as easily with a historical account of the past few centuries (as Greyhawk does it). You can easily make a campaign around it, but IME this usually leads to the end of the setting at the climax of the campaign. It's not a bad thing, but not something most DMs are going to run.The creation myths in official setting don't matter by design. D&D clerics don't really do much religious stuff by default. They are warriors sent by churches to crush enemies, protect flock, and bring back riches.
But it is easy to make a creation myth matter.
What is created can be destroyed or altered. And the processes can affect both immortal and mortal.
When I mentioned the priesthood, I wasn't talking about clerics, but the actual priests who perform rituals and services for the people (e.g. the acolyte background). Clerics (and potentially paladins, druids, and rangers) are specific types of holy servants granted the favor of the gods, as you rightfully pointed out. Not all priests would necessarily be granted spellcasting abilities.