The operative word was "rough". They give a few hints; they're not painstakingly mapping out the melodies and chords for every instrument and determining what those instruments are, and how they sound.
If you're writing for a guitar and voice, then you've basically got a song, but pop music is built on sometimes hundreds of different instruments and effects.
If you're writing for a guitar and voice, then you've basically got a song, but pop music is built on sometimes hundreds of different instruments and effects.