Fair enough, but I think Eberron does run off prayer=equals power. And at any rate the point is that the definition varies from setting to setting, and that trying to apply one setting's definition to another setting runs into problems.
I would suggest that given the very limited employment of all powerful gods, using omniscience or omnipotence(particular to the dominant segment of religions on Earth) is too restrictive of a metric, simply because they are not generally employed by writers.
We'd too easily fall into a semantic trap of being unable to discuss the gods of most settings by the term gods.
But there was Eru Iluvatar, the "Maker" of LOTR. With him in the picture the Ainur become demi-gods at best.
The existence of a higher ranking or more poewrful deity doesn't mean their Hades/Osiris equivalent, Mandos, less of a god any more than the precursors of Hades, Zeus and their ilk make them less of a god.
If the question is no longer, "do they exist and are they supernatural beings?", but "how powerful relative to other divines are they?" or "are they worthy of reverance", I think we're no longer in the realm of a character being atheist and more in the realm of a character not wishing to revere gods.
Say hypothetically, the Maker did create Thedas and is an omnipotent and omniscient god. Does that mean the elven gods wouldn't be gods if they were divine beings of lesser power? I'd argue no, they're still gods, much like gods in other fantasy settings are still gods despite not being the biggest god on the block.
Although we've yet to establish supernatural abilities and immortality for anything besides Corypheus and Archdemons. Additionally the level of power we've seen from either in game has only been that of rank 0 (demi-gods or quasideities in D&D settings). Though Corypheus's dialogue suggests the Old Gods of yore granted prayers, which in turn implies that they were beings of more significant divine power.
From the in-game discussion of the elven ones, they might well have also been prayer-granting portfolio/theme managing gods as well, up until they lost their powers somehow.
Of course, none of this is available to most characters, let alone most people in Thedas.