You definitely do not understand the word, because it is not limited to people, the word "insider" is limited to the information: is the information publicly available? If the answer is "no", then it is "inside" information, and trading on it is "insider trading".
These short sellers were trading on publicly available information. They just gave this information more publicity after getting their short positions, which is what you should do if you discover a market inefficiency that nobody else has discovered yet. That's your reward for making the market more efficient.
BaFin claims that this is insider trading was BS then and is BS now. They just don't understand how markets work, what their job is, and as a consequence did a poor job.