As we are told from the start, "narrative and form can bring us closer to the truth, but they can also be a weapon with a great power to manipulate". And that Catherine's work is to "cut through narratives and form that distract us from hidden truths". So it is stated that she has experience with manipulations and hiding the truth. It leads me to think that her side of the story was a lie, a clever manipulation that she decided to use as a solution to the extreme situation she encountered. The book exaggerated some things, sure, but overall I think the affair is a much more plausible version. Catherine concluded that no one can confirm either version as there were no witnesses and because it happened so long ago, so she decided to create her own narrative and present it as truth, to get back the power that Stephen was trying to take from her.

Besides, it would be a real shame if her version is the truth. If so, the show will completely lose it's edge of moral ambivalence and complexity. If what she says is the truth then the story becomes extremely superficial and preaching. I wouldn't like that narrative at all.