I think the perception is that "public" for internet privacy realists means someone might know that, not everybody can know that at will.

To say it another way. Private means, only I know it. And public just means anything but private. With varying degrees of how public something might be.

Funnily enough that's very different from what ordinary people think private and public is.

Private is everything they didn't explicitly intended to be visible to all the people on the internet and public is only that.