I've always thought ‘multi-document interfaces’ as we used to call them are an anti-pattern. I have a perfectly good window manager; why does every app need its own incompatible, usually inferior window manager built in? (Mind you on mobile I very much don't have a perfectly good window manager, and indeed can't even open multiple instances of most apps…)
Would you extend that argument to tabbed interfaces as well? Why should browsers support tabs (and an inconsistent interface by each vendor), when you can just open a new window instead?
So if you create a webpage that is so damn advanced that it beats the browsers OR it somehow reuses heavy resources within one webpage, I'd say this is a good justification. And IMO the OP link isn't an example of that.