In the writing challenge "The benefits of music" these sentences appear:
"Que ce soit le jazz, le rock ou la variété, il est indéniable que la musique fait partie intégrante de nos vies. Mais que nous apporte-t-elle qui nous soit si indispensable ? C'est bien connu : la musique adoucit les mœurs."
Why is "il est" applicable in the first bold phrase but "c'est" in the second? It appears to me that both are making general statements (about la musique) and both follow est by an adjectival phrase - not a noun, so I would think that case 2a applies in the A1 lessonn "C'est vs il/elle est: Saying it is".
Perhaps that rule is inappropriate here since "la musique" is not a "pre-mentioned thing" but what are the rules being followed here?