What I think the post you quoted meant was a classic song, not necessarily a classical song like, say, Mozart.I've never seen that movie, but if you mean classical music I've heard the same thing. I can't remember exactly the logic, but I think it was basically music increases your adrenaline, and when you are learning to fight you shouldn't be relying on your adrenaline for it.
A classic song is one that everyone knows and has been around seemingly forever. Like Ray Parker, Jr.'s Ghostbusters song -
When there's something strange, in the neighborhood
Who you gonna call?
I ain't afraid of no ghosts
The classic song gets played on the radio and used in ads, etc. for decades, earning the artist a steady stream of money.
Queen's We Will Rock You, Led Zeppelin's Stairway to Heaven, even whoever the morons are who wrote and sang Who Let the Dogs Out.