Author here. It’s definitely true that extending existing browsers is the fastest route to this kind of behaviour (though it’s not clear to me if it’s the best route). In fact, if you squint a bit (or maybe a lot), it could be argued that with application-specific URL schemes we kind-of sort-of already have the primitives we need to make something like this work.

Practically, though, if you want the multi-program side of this (which is kind of orthogonal to the 'multiple perspectives on how things are connected' side), then to make this kind of multi-window hypermedia system usable I think you need to have deep integration with the window manager. While Chrome OS tries to achieve this kind of integration by making the browser the OS, I propose that the best way forward here is to effectively make the OS the browser, as I discuss in the article. (Of course I’m not talking about the kernel when I say ‘OS’ here, but the desktop environment). At that point, I’d say the browser is different enough to the browsers of today that the description of ‘freeing the Web from the browser’ is still accurate.