She sometimes felt uncomfortably observed, as if living under the unblinking eye of a security camera. Florence wasn’t sure what she’d want to hide from her own son. Yet what best protected privacy wasn’t concealment but apathy—the fact that other people simply weren’t interested.
Am not sure if I'm parsing this right. My guess—there is an ellipsis: Yet what (she was sure about was the fact that) best protected privacy...and so on. But if so, then shouldn't it be Yet that, not Yet what? What here means "the thing which" and it doesn't make sense if we do such a swap in the original sentence: Yet the thing which best protected privacy...??? But it is OK if we use that, not what: Yet (she was sure) that best protected privacy... What do you think of this?