Most of the human moods, energy levels, immunity, recovery, consciousness can be controlled by chemicals. It is no surprise that continuous use of a chemical will have impact.
The worry always is what does that extra chemical destroy as the body doesn't produce it naturally.
Creatine is one of the most studied substances that humans consume.
Creatine has been tested over long time spans already so this comment is not applicable here.
The better rebuttal is that the paper didn’t follow RCT protocols.