On their first night at the Leadership Camp run by their new-found, anti-vampire religion, Luke got angry at his room-mate, Jason Stackhouse. Luke didn't like it when Jason broke the American flag pole in order to make a stake to kill a "vampire". Especially since it was all during a play-acting exercise.
Because of the incident, Luke accused Jason of being a "Muslim Buffy with a dick".
I've got no problem when 'Buffy The Vampire Slayer' is referenced in a TV show. Just so long as it's a reference to "Buffy The Vampire Slayer" movie, that is. This way, they aren't sharing the same creative universe; one is in the TV Universe, the other in the "Cineverse". As to why there would be a movie about Buffy Summers, again it all depends on how much is mentioned. So long as nobody states when the original movie came out, then we can claim it was made after the events of the final episode of the TV series.
Otherwise, it's time for the Splainin Dance.
So in this case, since Luke didn't even cite the entire title of the show/movie, there's no Zonk to be staked.....