Brownie,
Disbelief - doubt without necessarily understanding
Unbelief - rejection of a belief
Non belief - definitely not believing.
Sort of.
Disbelief - actively thinking something to be false.
Unbelief - the state of not having a/the belief that something is true.
Non-belief - the absence of a belief (eg, atheism).
Unbelief and non-belief are more or less the same thing. Sassy's mistake was to think that unbelief/non-belief are also the same thing as disbelief. For practical, everyday purposes atheists do proceed as if we are disbelievers - for exactly the same reason that Sassy proceeds the same way regarding her disbelief in, say, leprechauns - but in strict epistemological terms we cannot say that gods categorically do not exist, again for the same reason that Sassy cannot say that leprechauns categorically do not exist.
What can be said though is first that there's no evidence for gods/leprechauns, and second that those who argue for gods/leprechauns have only logically false arguments to deploy.
That's not to say that at some point either evidence or a cogent argument for either gods or leprechauns (or both) could not emerge, but it is to say that there's neither so far - hence unbelief/non-belief rather than disbelief.