I freely admit I have no grasp on what "suavity" means, and I didn't think DeWitt adds anything to explain it. He just assumes I know what it means and keeps using it.
Being suave or having suavity didn't seem all that positive from what I was seeing in various dictionaries. Without looking anything up, it conveyed being smooth, cool, or slick in The Fonz sort of way. But I read some definitions as being pleasant to the extent of being insincere. Wiktionary at least has "The quality of being sweet or pleasing to the mind; agreeableness; pleasantness" which doesn't help a lot for me.
Bryan 's bringing up χαρις is much more helpful to what Dewitt is trying to convey.