What people don’t understand is that satellies don’t run traditional OS. They run simple state machine firmares and real-time software. Also, you don’t interact with them with something like a command line and general shell commands. So even if a bug or vulnerability were to be found in some library, driver or firmware and it’s far from a vector for attack.
A way more plausible way of gaining control is to attack the ground equipment. Anothr method will be learning the command and telemetry dictionary by careful observation of the communication (not trivial, but lets assume possible).
Now you have the problem of what to do with the control. The usual fanciful doomsday scenario is diverting the satellite to hit another one. this is extremely unlikely, manoeuvring satellites isn’t like making a car take a turn, it take careful planning and execution. Even then it’s not fast, the other satellite operator can see it coming and do a small manoeuver to be able to dodge. On top of all that, lots of nano and small satellites have a very limited propulsion system if at all.
The other alternative is to hurt it’s ability to operate, which is a way more easy. But again, there are simpler and easier ways to that - mostly by jamming and disrupting communication.
In conclusion, not that I think cyber security in satellites shouldn’t be a thing, but that article, based on an academic study of the code of 3 firmwares from amateur to research level small-sats, doesn’t impress me much.