I kind of take this point like I take Sraffa's critique of Hayek or certain criticisms of marginalism: it's a good reminder to avoid sloppy talk.
I think we can make statements about the public good, we just have to recognize two things:
1. There is no actual actor that is the collective, and
2. What we identify as the public good is of necessity normative even if it is informed by everyone else's utility function.
So I would feel perfectly comfortable talking about the need for an egalitarian society and potentially legislation to achieve that as being in the public good or even an American ideal. I might use language Don would consider collectivist.
I can make the claim quite legitimately that I come to this conclusion based on some sort of aggregation of what other Americans think. It's a rough, back of the envelope aggregation on my part, but it's legitimately an aggregation of individual preferences.
But it's not some sort of objective, uncontestable aggregation of those preferences. Instead it's an aggregation based on my some of my own values as well (which, in fair divinations of the public spirit, is itself derived from public values and virtues). This isn't a useless way of talking about the public good, but it isn't objective either - not in the same way that my preferences are (they are subjective of course, but I can objectively say "Daniel prefers X").
Similarly when we get an election result it's tough to say that it is the public's will. What we have to recognize is that it is one aggregation of individual preferences, and the aggregation is done according to the dictates of a certain set of democratic institutions.
THAT is a fair, objective claim about what this "public will" is.
As Ken Arrow and Don Boudreaux discuss, it's not legitimate to talk about this as an actual sensible preference set on its own. It isn't.
As individuals and as citizens, what we have to decide is whether those mediating democratic institutions aggregate preferences in a way that we like or not.