Atheism is not the denial of a SPECIFIC supreme being, its the denial of ANY sort of supreme being. Agnosticism is the denial of a specific supreme being. The two are not the same and are not interchangeable.
What is someone called that just doesn’t know if there is a supreme being or not?
There are actually two "flavors" of atheism. Strong atheism denies any sort of supreme beings as per your definition: "I know there isn't a God or Gods", whereas weak atheism is simply a lack of belief: "While I can't know that there isn't a God or Gods, I see no reason to believe that such a being(s) exists."
As for agnosticism, I think your definition is way off. The way you define it Christians would be agnostics, since they deny all sorts of specific supreme beings (Zeus, Odin, Krishna, etc.).