I consider them so because they are not simply sharing a belief, they are trying to purposefully tell another group of people what they believe in is fake. Why go around telling others "there is probably no God", if you do not believe in something why broadcast it? It seems they are merely doing this to try and get a rise out of Christians. Then there is the part about "stop worrying", they are trying to tie a negative connotation to the belief in God. However, I believe this matter really comes down to opinion.
Ah. I took a completely different meaning from it.
To start with, because I knew it was a response to those Christian ads, I figured that the intended audience was the same: i.e. non-believers (mainly non-Christians) that Christian groups are trying to convert.
I don't think the intent was to rile up people who are set in their faith; I think it was more intended to ease the mind of people who had now started to worry that they might be hell-bound for not believing in Jesus or the like... potentially also people who felt duty-bound to stay in a church they didn't really believe in.
IOW, I think the ad was directed to people who
are worrying, just as the ad itself suggests, and it tells them to stop worrying and why.