Marx called religion an illusion and the opium of the people. Therefore these views are Marxist.
Just because he wrote that, doesn't mean that everyone who thinks religion is unfalsifiable is a Marxist.
That would be like saying a black man said Popeyes fried chicken is the best, so every white man who says Popeyes fried chicken in the best must be black.
Marxism is not just about the claim that religion is an illusion.