Why would you assume the uplink isn't authenticated and/or encrypted, given that the downlink data has very little value but interfering with the uplink could disable the probe?

Spacecraft are power and compute limited. It's entirely possible the designers chose not to spend the resources to encrypt a downlink that isn't sending anything that needs to be kept secret. In contrast, it's reasonable to assume that the uplink is at least protected by a HMAC, because the consequences of an adversary exploiting the uplink are so great.

The Chinese are well aware that the US security establishment is petty enough to interfere with a scientific probe purely for its own amusement. They will have taken precautions to deter this.