> Any literate German can read the NZZ easily, but they cannot have a colloquial conversation with an average person from Zürich, unless the latter switches to standard German (which is a foreign language for them, though one they have to learn from age 6).

I presume they also pick up a lot of standard German in the media: there's lots of German movies, and Germany has the biggest movie dubbing industry in the world, too. There's some Swiss German media, but not nearly as much as there's on offer in standard German.