It is also possible that a country can spy on traffic that isn't routed through that country's territory. Three things we've heard about in the last few years have been bilateral deals between spy agencies to share intercepted data, compromise of routers, and installation of clandestine physical taps, whether on land or undersea. There might be good political remedies for some of these things, but at the technical level it seems like Joseph Hall's view (from the article) is exactly right.

Edit: there were also reports about different kinds of routing manipulation, such as BGP attacks, inducing ISPs to deliberately choose surveillable routes in some cases, or inducing link outages.