It is hard to argue with someone who creates their own definitions for words. For clarity: theism is a belief in a god or gods[1]. Atheism is lack thereof, both in modern usage and normal latin meaning of the a-prefix. (Ideally of course atheism would not need to be distinguished separately - only theists vs. everyone else.)

Agnosticism asserts that the existence or nonexistence is unknowable[2].

[1] http://dictionary.reference.com/browse/theism [2] http://dictionary.reference.com/browse/agnostic