Of course living in Regency England instead of 21st Century USA Mr. Knightley didn't worry about stereotpying people. I think he means that French meaning of amiable is all about courtly manners and pretty words, as in Frank's letter to Mrs. Weston. But the English are more concerned with being kind and a sense of noblese oblige. That is how I read it anyway. And despite generations of French ancestors I find no fault with Mr. Knightley's statement, because I can never find fault with Mr. Knightley.