I don’t get the Philip Glass reference.
Just a non-sequitur kind of joke.
Sing-along music is usually based on very simple, very direct themes. Row your boat, Bingo was his name, etc. Philip Glass’ music is extremely indirect – it’s called “minimalism” for a reason! – and is about the last thing in the world you’d want at a campfire. If you’ve listened to it, you’d find it pretty close to un-singable.
It could just as easily have been Leonard Bernstein and his “Mass” or Sergei Prokofiev and his “Fifth Symphony.” Pretty much the same joke.