MOST (but not all) people who call themselves "Atheist" try to make it clear that they are NOT saying there is NO POSSIBLE GOD(S). Instead, they're simply asserting that there is no compelling evidence of any SPECIFIC GOD(S).

Exactly my problem with them. You do not get to 'make clear' that the definition you operate under, isn't what you actually operate under. Then don't call yourself an Atheist.

Atheist always fall back on 'AGNOSTIC' criteria when it suits them.
No, ATHEISM IS THE REJECTION OF ANY GOD.
If someone isn't sure, or just says, 'well there is no proof I don't know' then they are agnostic. To take the FAITH based belief that 'there is no creator' is just that, FAITH.

I mean, how much "faith" does it take to NOT-believe in Santa Claus?

Fallacious. Santa Claus is not claimed to be your Creator, so why would you compare Satan Klaws to a the supposed God? You are ALIVE. That is where you start with 'is there a God'. You don't go there with Santa. You don't think to yourself "did santa create me?". You know, that humans 'made up' Santa, so the comparison makes no sense. You don't 'know' that humans made up a creator. We are here aren't we. Something made us be here, unless, you think "Accident+Evolution" ;)