Basically, the author is smugly amused that the bus ads say that there probably isn't a god rather than saying that there definitely isn't one. In a blinding display of brilliance, he assumes that this indicates a lack of conviction and "demonstrates the weakness of the atheistic argument" (The irony is that if the message was more along the lines of strong atheism, these people would just accuse atheists of being dogmatic. Smug dismal either way.)
Well, as it turns out, the real joke is in the comments. The author didn't do the research and some reader kindly pointed it out to him.
But the mere fact of not understanding what they're talking about sure doesn't stop idiots from repeating the talking point. (who themselves are kindly corrected in the comments)