The bottom line is exactly what difference does it make, if the plan was to detonate explosives over the Atlantic ocean? Sure, the destinations were symbolic (major American cities), and Boston, having been one of the departure points for the 9/11 terrorists, would probably be on the list, but again, if they didn't intend to reach their destination, the destinations aren't all that relevant.
The fact that American citizens would die, and American aircraft destroyed, OTOH, is very relevant.
You missed part of the post. According to that, the detonations were to take place over the target cities, not over the Atlantic.
If the planes exploded over a major city, or at the airport when landing, there would be truly stunning loss of life and economic disaster for that city. I think some smaller cities thought they were immune, not "important" enough, but the truth is that any city here is important.