"even cats and babies could be atheists"

The alternative is call them theists. You're either one or the other – it's a binary position. Since babies and cats are not theists, then logically they're atheists. WLC just calls this ridiculous, but doesn't say why.

Shinyblurry: "If you are unwilling to say God does not exist, you are an agnostic and not an atheist"

A/gnosticism deals with knowledge, A/theism deals with belief. They are two separate subjects. I can say I don't believe in aliens visiting earth without claiming any knowledge on the subject. I could be wrong, but as it happens I don't believe. You can say "I don't believe aliens visit earth, but I could be wrong". You CAN'T say "Aliens don't visit earth, but I could be wrong".
The first sentence is coherent, the second is self-contradictory. Thus it's pretty obvious that belief statements and knowledge statements are not the same.