I was bothered by this as well. The statement wasn't "cocktail parties and salons are patronised by the ultra-rich", but "the ultra-rich ... spend their time at cocktail parties and salons". So it's as you say, what you need to look at is not the guest lists for cocktail parties and customer data for salons, but what proportion of a typical ultra-rich person's time is spent at cocktail parties and salons. I don't have the data, but I'd anticipate the mean ultra-rich person spends more time managing their business concerns than attending cocktail parties.

Though this is all Support That Sounds Like Dissent, since none of it really detracts from the central thrust of the post, with which I broadly agree. Still, no point leaving holes in something which is political enough for people to have a good deal of motivated scepticism about it.