She’s a Russian Jew! Ayn Rand is her pen name. Her actual name is Anna Rosenbaum. So why would she know much about Christianity? As a Russian Jew her and her family’s experience with Christianity (Russian Orthodox variety) is not likely to be positive.
She wouldn't, not unless she studied it, which people of any background can do.
My point is, a rational person knows what they don't know. But when it comes to Christianity, it seems that every non-Christian feels competent to comment on it without studying it. Atheists do so all the time.