Is that MP3 actual raw recordings, or has it been "enhanced" with interesting interference. I'm hearing lots of echo, telemetry, different voiced "layered" on top of each other, etc.
There is a lot of interesting interference at these low frequencies. I didn't hear anything I wouldn't expect to hear on HF. Also, echo is a complex and somewhat mysterious topic. Long Path echo is easily grasped; the same signal is received twice due to two paths around the Earth which happens when band conditions are favorable. Other forms of echo (Long delayed echo[1], for example) are less easily explained.
[1] https://en.wikipedia.org/wiki/Long_delayed_echo
This USAF HF system uses a lot of power (4+ kilowatts) and very good antennas, so they're prone to experience these effects.