Opera, symphony, etc weren't the affairs we see today throughout their history: the quiet sterility is a modern behavior — and my understanding that it used to be quite a bit more like, eg, movie premiere crowds that made noise in response to the show.

I think the emphasis is on the class structure, formality, etc. rather than saying the Coliseum followed modern theatre etiquette. And the according comparison about status of attendees, etc.

Yes, Bernard Shaw used to complain in his reviews that no one listened to the music and constantly talked, as concerts were a social event, and you promenaded around.

SOme history here too https://ledbooks.org/proceedings2019/tag/silence/

So I think this backs my point - when she's referring to Opera, she's referring to a modern conception of opera, not an 18th century concept of opera. I agree that's more normal sounding.