This all assumes constant, good and cheap data network coverage tho - is that an assumption the whole OS is making? :-( if so.
All you need to get the offline experience with an App in this situation is having an Install button that prevents cache eviction.
It also mentions P2P retrieval, which would further avoid using an expensive data connection.
It sounds like with Fuchsia any predicability goes out the window. I could do a task one day that is fine, the next day it causes issue. My question is, does the user get any predicability and control? Eg: you mention an "install" button - will there be one? The article doesn't mention that. In fact, it's the reverse: "The most important thing though, is that these processes will be completely transparent."
(Admittedly knowledge of this kind of thing does require some technical skill on the part of the user. Maybe Google have decided to remove that to make a more user friendly experience. That might be a good choice to make, but seems worth commenting on at least.)