Their paper doesn't identify where the signals came from. "The emitter source remains an open question, but could range from covert ultrasonic exfiltration of modulated data to ultrasonic jammers of eavesdropping devices or perhaps just ultrasonic pest repellents. It’s also possible that someone was trying to covertly deliver data into a localized space using ultrasound to say, activate a sensor or other hidden device."
It's better and more careful not to over claim.