That they needed to use balloon-borne instruments to collect this data tells us something about their satellite intelligence-gathering capability.
Our SIGINT satellites use dish antennas that unfold to great size in orbit. I’m sure that’s not the only advanced capability they have.
Perhaps the Chinese are not able to copy this technology, so they have to get their receiving equipment closer to the sources in order to get useful results.
All information in this post is from publicly available sources.
Maybe to receive certain signals effectively, they have to be at lower altitudes than what satellites fly at.