The story about their horses dancing to music played by attacking enemy is probably a fable.

If a horse can prance to music, who says it can't dance to music, wwh?

Or perhaps you are only skeptical about an enemy getting a Sybarite's horse to dance when it should be advancing.

That would be a comical sight. [Not comical to a Sybarite, of course.]

Personally, I am skeptical about the story, not because I think a horse trained to dance will advance, but because I think a Sybarite would not be aboard any horse that had any inclination to advance.

A Sybarite would have the good taste to leave the fighting to others.