It’s not true, but the way it plays out makes people think otherwise.

There are tons of first gen atheists who came from deeply fundamentalist families, and often believed everything well into adulthood. To name two examples, Bart Ehrman and Genetically Modified Skeptic.

What won’t happen is changing someone over the course of a single debate. They will come up with all sorts of mental gymnastics right on the spot. However, conversations like that do add up over time. An otherwise inquisitive mind will find a way eventually.