But it's not about Wesley's inability to tell them apart. Mendon specifically says that they do all look the same. It has nothing to do with the fallacy that not being able to tell one stranger from another makes you a racist if they are from the same ethnic group.
Mendon says "we're from the same geostructure, naturally we all look alike." Which, pretty much, is saying "we're all from the same planet, you idiot, of COURSE we all look alike!"