If you change the conditions of the race, you can just as well make it say just about anything.
If you give the pigeon a 512 KB message, and an identical 512 KB message to be sent via a rural broadband connection, then the rural broadband connection will win.
If you give the pigeon a 64 GB memory card, then you could say that the pigeon has a transfer speed equivalent to 104 mbps, which'll mean it's faster than most broad connections, rural or not. (Assuming an average speed of 60 miles per hour for the bird.)
You're totally on the mark, but you could take the limitation you mention and turn it into a better test. For example, instead of just asking, "what is the latency to transmit a 200MB file across 84 miles?" You could ask, "What is the relationship between message size and latency for pigeons and copper wire (or, whatever they're using) over 84 miles?" This latter test would involve transmitting files of several different sizes.
I'm betting that, at some point, the curve for pigeons will either asymptote or turn over (e.g., when the mass of 32GB microSD cards begins to exceed the flight capacity of the bird).