“Slander” is a loaded term, too. It isn’t only speaking falsely about a person, but doing so in a way that makes them out to be immoral when they aren’t.
If someone claimed that Dr. Mouw’s wife was forgiving and gracious to all people who have done her wrong, and Dr. happened to correctly know and believe otherwise, I expect he would in fact reply much the same way as here: thanks for the compliment, but she isn’t that forgiving and gracious to people who have done her wrong. Yes, she can be very extensively and amazingly gracious sometimes to some people, depending on her whim, but everyone needs to be aware that in regard to other people she is not gracious and forgiving at all. And (so Dr. M believes) his wife is entirely within her rights to be that way, and has good reason to be that way! It’s understandable that Rob feels so strongly about his wife’s mercy and graciousness, since she was gracious and merciful to him after he did her wrong, but honesty constrains Dr. M to point out that his wife will certainly not be so merciful and gracious (or maybe even at all merciful and gracious) to other people who have done her wrong.
What Rob would be claiming to be the real truth about Dr. M’s wife wouldn’t (per hypothesis) be true, but it wouldn’t be slander per se either.