I just tested and it seems you're right.
Our source code uses a SpatialIndex class (from an external library) to keep track of robots, and we call the
nearestNUnsorted method to return the list of nearest robots. This method specifically has no guarantees and doesn't try to sort anything, but perhaps due to the underlying data structure, it ends up processing the robots in distance order (at least most of the time).
We can't guarantee we will keep senseNearbyRobot's behavior this way, since it is not documented. Follow the issue here: https://github.com/battlecode/battlecode-server/issues/379