The Juno spacecraft will receive the message on multiple frequencies so I suspect they'll confirm based on QSL contacts. I tell them what frequency I worked and what time, they correlate it against the log to see if it was received and decoded. When I would ID it was a lot faster than 1/25th WPM they wanted the message. Since I was using an autokeyer it was sending my call sign at 25 WPM, so 625 times faster than Juno wanted. When you decode background noise, say you're doing RTTY or some other non-CW data, you'll get a slow stream of random characters at maybe one or two per second. It does not have to be high speed at all to mimic that.
Even if it could process and decode call signs that fast I doubt they'd want to dedicate the storage to do anything with it. They were calibrating an experimental instrument, so it's not so much about receiving detailed information because I believe the device is designed to study EM properties of the Jovian (gaseous) atmosphere. It's learning about the wave properties themselves, e.g. the carrier so to speak. Think of it as listening to the atmospheric white noise on HF trying to understand what it's telling you about the composition.
It's kind of cool because I worked on the bus for Juno, one of the C&DH ADC cards that they use to fly the spacecraft.
"Stand your ground. Don't fire unless fired upon, but if they mean to have a war, let it begin here." -- Capt. John Parker