The video shows a moving spectrogram of the received signal from the TPL-25 towed pinger locator. It has nothing to do with sidescan sonar. In fact they explic

The video shows a moving spectrogram of the received signal from the TPL-25 towed pinger locator. It has nothing to do with sidescan sonar. In fact they explicitly have not launched the sidescan capable Bluefin21 AUV because of fears of acoustic contamination of the towed pinger audio.