The click-bate title'd Space News article China quietly used NASA’s Jupiter probe to test its deep space network contains the quoted material below.
Everybody listens to everybody else's spacecraft's transmissions all the time in fact: Hijacked space data, notable instances of recovering images or other goodies from someone else's space mission? and there wouldn't be data in JPL's Horizons for some spacecraft if not for amateur trackers listening to the probes on the way to mars, decoding them and sharing the state vectors they contained (yes!) with the world (see this answer to Will the Emirate Mars Mission, Tianwen-1 and Mars 2020 Mission get closer to each other or spread out on their way to Mars?)
While the Chinese team successfully acquired signals from Juno and determined its orbit through subtle Doppler shift measurements, these actions went under the radar.
In response to a request from SpaceNews for comment on the test and if coordination has taken place, NASA Jet Propulsion Laboratory spokesman David C. Agle stated that the agency was not engaged in the activity.
Brian Weeden, director of program planning at the Secure World Foundation in Washington, told SpaceNews that the tests will be helpful, but don’t pose any threat.
“This is like a ham radio operator trying to tune into the most distant signal they can to test out the sensitivity of their equipment. Juno is broadcasting its carrier signal back toward the Earth and it’s an open signal, by design. Anyone, even hobbyists, who happen to have a powerful enough antenna can pick it up.
Now if Juno's signal was performing a secretly coded frequency hopping over a very large range of frequencies, or doing secretly spaced extremely wide band but short time-domain bursts so that it would be hard to "pick up" if you didn't know in advance where to look, it couldn't be tracked by DSN with range-rate and VLBI ΔDOR techniques. How do we track the exact location of the spacecraft which is millions or billions of miles away from us? So the spacecraft transmitting a continuous signal that any powerful antenna and a receiver could "pick up" is not really optional or by design; it's got to be that way.
Question: So I'm wondering if what "open signal, by design" really refers to is the modulation and encoding of the information within Juno's signals that is open?