If I understand Pemerton correctly,
he is distinguishing between polytheism qua ‘objectively existing’ tyrants,
versus ‘subjective’ cultural constructs.
The subjective kind of polytheism is more about symbols and interpretation and ways of organizing experiences of the world.
It is the difference between actual polytheism and the idea of polytheism.
So, the descriptions in the Conan setting seem to him to be moreso cultural constructs. In other words, human-sourced.
But, again, this is a bit of a red herring. In Hyboria, for example, gods are, by and large, pretty removed from the setting. You don't have gods making appearances. You don't have spell casting clerics a la D&D. Thulsa Doom gains his power from Set. At least, that's what the books say and there's nothing to contradict that. While, true, he's called a magician, that's simply because the concept of a D&D style cleric didn't exist then. Everyone who used magic was a magician, regardless of where the magic came from.
Now, D&D (and frankly, most fantasy settings) has actual gods in it. This isn't some new concept - Supplement IV was published only a few years after Chainmail. The notion that D&D is polytheistic is pretty long established. The alignment system, the cosmology, and, well, big honking books full of gods plus the fact that every single setting published kinda points pretty strongly toward the idea that D&D is polytheistic.
So, yeah, you have your work cut out for you if you want to use D&D with a monotheistic setting where there actually is only one deity in existence. The closest you might argue would be Dark Sun, although, again, that's not really true since druids simply worshipped forces. They were animistic.
What we're left with then, is what has to change, mechanically, in order to have a single deity setting. Really, quite a lot. You need a completely different cosmology (since it doesn't make a whole lot of sense to have fifteen different Heavens when you only have one god. Clerics aren't too bad, all you have to do is outline what a cleric actually is in that setting. Paladins are probably easier since if you eject Oath of Ancients paladins, you're probably good to go. Other than that (and that's not to say that that's a small change) you're pretty much done.
Me, personally, I'd probably change demons and devils to be a single type, probably avoid the elemental types for the most part and focus the campaign on more mundane type critters - humanoids, beasts, and that sort of thing.
It's not impossible to do. But, I would think that you're really going to have to have a lengthy Session 0 with your group in order to make all this as clear as possible. I know I've run into this sort of thing from time to time. When I ran Scarred Lands, I had a heck of a time explaining to a player that no, he can't play an Elven Cleric. The god of the elves is dead. That's why they cannot have children and all current elves are the last of their race. That's the whole point behind elves in Scarred Lands. It was an uphill battle.