I think the “joke” (quotation marks are working extra hard here) is that anti-theists want to get rid of all religion, so they’re rooting for the Christians eradicating a bunch of religions.
It’s likely a misrepresentation of antitheism, but edge lords be edge lords.
Western atheism and antitheism often exist as reactions to Christianity on a factual or ethical basis, while remaining within a Christian cultural and moral context. Historically, Christians and atheists have worked together to attack other religions, such as in the case of the stolen generations in Australia or the cult panic in America. This pattern of behaviour is counterintuitive if you're culturally Christian (as most atheists are), but from a broader cultural perspective it's kind of obvious. Christianity and white atheism exist as offshoots of the same history in the same way that Catholicism and Protestantism do.
If you have an actual understanding of pre-roman polytheism, then you're capable of seeing the difference between belief and worship. A difference Christians have tried to erase, and white atheists have not challenged. Giving up cultural Christianity is beyond most white atheists' ability to even imagine. What I call cultural Christianity, they would just as soon call "common sense" or "reality".
Also I've never met an antitheist who has spent longer than an hour thinking about whether they're advocating cultural genocide of indigenous people.
I agree that modern and western atheism is sort of a dialectal to western religion and so perpetually tied to it in a way, that westerners don't often see.
I agree that atheism as commonly portrayed online (actively and by its detractors alike) is militantly against Islam, all religions, etc.
But lots of atheists are actually humanists and not militantly out to root out all religion entirely.
Inside the US we are plagued by a religious fascism that yes, sadly, colors a looot of the online conversation about faith and atheism.
Atheists are not generally for eradicating all religions on the planet.
I agree with this in general, and wanted to add onto it a bit:
I'm Atheist but I honestly couldn't care less what others believe in. If believing in a god or an afterlife helps you get through your life easier, than why not? Why try to tell someone else what religious or cultural belief is right or wrong when we're all just making our best guesses anyway?
Believe in whatever afterlife you want, so long as that belief doesn't make everyone else's lives worse. The moment you start trying to push religion and culture onto people, you're no better than any other form of cultural imperialism.
Western atheism and antitheism often exist as reactions to Christianity on a factual or ethical basis, while remaining within a Christian cultural and moral context. Historically, Christians and atheists have worked together to attack other religions, such as in the case of the stolen generations in Australia or the cult panic in America.