So this may be visible on the spectrogram, right?
A real world example is GPS, which uses a spreading code to provide about 30 dB of gain. GPS signals aren't directly observable relative to the noise floor in many receivers. It's not until after the signal is "de-spread" that it becomes observable in a spectrogram. This process requires prior knowledge of the signal structure.
In short, if you don't need to send data at high rate there are many ways to hide your signal.
We're also dealing with only 48KHz, which is 2-3 magnitudes less than what SDR people are accustomed to dealing with.
And we can also make our own detection systems by having a 3 person "meeting" with fake audio, with this fingerprinting on, and then comparing the 3 recordings.
Link to the video? Searches aren't coming up with anything useful.
https://www.youtube.com/watch?v=z9RkPt6uVQ4
Note, it's not from the official Def Con channel, so some fuckery is afoot.