IANA optical communications engineer, but this seems kinda wrong. Saying a radio wave is at 500KHz with a bandwidth of 5KHz does not imply any level of spectral efficiency unless I greatly misunderstand spectral efficiency. Discuss?
Indeed, you are correct. Spectral efficiency is, essentially, how "well" those 5 kHz are being used. It doesn't have anything to do with the center frequency (500 kHz in this example).