Yep, Luke is the epitome of not being able to see a person's expression over the internet. Also keep in mind that he's from Australia or something with a similar accent, so there's also that barrier.
I believe the "racial slur" was him mentioning "those afro guys" when referring to some of the guys down in the L.A. area. I really don't think he meant it in a bad way because he seems like a live and let live type of guy. Probably just didn't have the right words, but then I may be mistaken.