The actual words here are, “I’m an atheist because I refuse to pretend to know there is one.” That statement explicitly disclaims belief, and refers only to the speaker’s individual state of mind. It doesn’t ask or tell anybody else to prove anything. The speaker’s own subjective state of mind is actually the one thing that the speaker is qualified to make definitive statements about. “I don’t believe in God” needs no proof. Similarly, “I believe in God” also needs no proof, as it’s a statement about the speaker’s state of mind; “God exists” is a claim about the universe/reality that would require evidence.

The user zenithoclock, as such, has misinterpreted the statement as a positive claim about the universe. As for why, I don’t know for certain, but it does seem that quite a lot of people who believe in God can’t abide and feel threatened when other people don’t.