It wasn’t a “joke” He was serious and he’s also a huge POS. Rob Flynn had this to say about him:

Robb Flynn was at Dimebash 2016 to perform alongside a host of talented musicians and pay tribute to late Pantera guitarist Dimebag Darrell. Flynn claims there was no white wine backstage, tossing out Anselmo’s “inside joke” defense. “The only thing you were drinking, Phil Anselmo, was Beck’s German beer. Maybe that’s where the joke came from, huh?”

Flynn recalls running into Anselmo backstage at Dimebash and dealing with another racially charged comment. “Within 30 seconds of sitting down and talking to him, his drunk ass decided to let me know in no uncertain terms that he hated the ‘nig-er’ era of Machine Head, referring to our third and fourth album, The Burning Red.”

F@ck Anselmo.