There's no preview. There's an icon and a page title, which only appears in the share menu.

See the addendum to the blog post: "The only purpose of the HTTP requests in Safari's share menu appears to be to display the link's icon and title in the share menu. Crucially, that information is not passed along to the other apps!"

I actually wrote a Mac app with a share extension, by the way (it's in the blog post screenshots), so I do know what I'm talking about: https://underpassapp.com/LinkUnshortener/