I suspect the loading coil has an extemely high Q tuned for only a few KHz of workable bandwidth at a station's set operarating frequency. Probably just enough bandwidth to keep the SWR curve within the transmitter's tolerance.
The top loading is interesting from a space saving standpoint, but assuming apreciable horizontal space is available, dramtically increasing capactive top loading should substantially improve performance. Once upon a time, there were AM BCB stations using such T antennas. Building a couple of shorter support towers could be more affordable or practical than the skyscraping towers we normally associate with mediumwave broadcasters.