She's not excusing cheating but acknowledging that it is merely one problem that can occur, among many problems.

Cheating is not the worst thing a significant other can do and just because someone cheated doesn't mean the person who was cheated on didn't also do terrible things as well.

Cheating is most often a symptom, rather than THE problem.

"But the way to address issues in a relationship is not by cheating but by communicating ... "

And the best way to address an issue in a relationship is not by pushing them away and shutting them out, but by communicating and, if necessary, ending the relationship.

You're stuck on the cheating thing but there were clearly other problems being created that spiraled into a symptom of the main problem, which is a lack of communication and honesty about the relationship.

That's the author's point. Unfortunately, you missed it.