Zizek's argument, basically, is that he can infer the defining characteristics of atheism from trends he sees in atheists.
Think about all the Muslims you know: the opinions they express, their behaviours, their likes and dislikes, etc. You'll probably be able to find lots of things that are generally common to most of them... right?
Now think about all those commonalities; how many of them are actually defining characteristics of Islam? Probably not all of them, right? Zizek's arguments would imply - if he applied them consistently - that
all of them are integral parts of Islam.
There's an episode of the Irish TV show Derry Girls where
they list similarities and differences between Catholic and Protestant Christians. Most of the differences they list are things like "Protestants love soup" and "Catholics love bingo" and and "Protestants keep their toasters in the cupboard" and "Protestants support sports team A while Catholics support sports team B."
A reasonable person can recognize that these are all generally true (at least in Ireland where the show is set) without it meaning that there's some tenet of the Protestant belief system that says "you must keep your toaster in the cupboard." But Zizek's argument - i.e. that we can infer the belief system of a group from what the group members commonly say and do - would suggest that where a person stores their toaster really is a defining aspect of these belief systems.
IOW, Zizek - despite being a philosophy professor - can't understand that there's a difference between a correlation and a fundamental property.
We can all recognize that this approach is nonsense when applied generally. It would be nice if the people here understood that it's still nonsense when applied to atheists.