We're arguing about technical power levels and relative rank. Not their existence, or their possession of super powers.
Widespread acceptance of the existence and even the direct regular intervention of a deity in the setting doesn't mean its impossible for a character to be an atheist. However, that more or less also requires either denial or ignorance of relevant divine interactions.
Specific to DA, we have yet to establish interaction of a divine nature(although this isn't what's being debated at the moment, we seem to be arguing over how powerful a deity needs to be to be considered a god), and existence isn't common knowledge either.
I'm making the argument that debating technical power levels and relative strength already falls outside the realm of atheism, from a metagame perspective there exists no merit in saying "This isn't really a god because he's only able to level cities with a wave of his hand, gods are restricted to continent level destruction and above". Once we've accepted that some being is supernatural in nature, it doesn't make sense to call it something besides a god.
Likewise in character, if a denizen of Thedas accepted the notion that some elven god of halla burns down villages with lightning bolts when he's feeling bored, regardless of worship of said being, or worship or acknowledgement of a more powerful being, they've stopped being an atheist.
Your logic is flawed.
A supernatural being in high fantasy world is different from a supernatural being in real life (if it existed).
A supernatural being in high fantasy is not a god by definition. I mean, my rogue can make his entire group invisible, is he a god?
In a world filled with dragons and liches and demons, acknowledging that there are powerful beings does not make you "stop being an atheist".
If anything, the basis of religion in fantasy worlds is much harder to explain that in real life, and mostly it has less to do with "power" and more to do with creation and "a personal god".
In a fantasy world, drawing the line between what would make someone an extremely powerful mage or an ascendant or a god...?
It's basically not even worth arguing over.
BUT...
Andrastians pray to a personal god, a god who watches their every move and action.
An atheist, in Thedas, does not deny the existence of extremely powerful beings. (they obviously exist, does not mean they're gods)
But he does deny the existence of a god who created the world or who watches over you all day and night etc.
...
For anyone interested in this, I recommend Scott R Bakker's books, The Second Apocalypse series.
First book is called the "The Darkness That Comes Before". It's a wonderful series, philosophy mixed in with fantasy, about a founding of a religion in a fantasy world.
Hard to get into, but once you do, it's fantastic.