You can’t tell anything from it because they once again just surveyed their own students by email.

There is no sample size and there is no response rate. There is also no control over having multiple responses from the same person or possibly from people outside of whatever the sampling population was (not that they tell you what that was exactly).

People who have abberant behaviour (like drinking 8 cups of coffee a day) tend to also have high prevalences of other unusual behaviours as well. A cross-sectional (almost retrospective actually) observational self-report study with a sample size of 219 which fails to control for more than the smallest handful of confounders does not allow much in the way of conclusions.

But to be fair to the authors they do not make strong statements about their findings.

So basically some university students (possibly only psychology students from one university) in England who don’t smoke show an association between a psychometric measure and another psychometric measure.

I know dozens and dozens of people who drink this quantity of caffeine. If there was any legitimacy to the claims at least 1 of them would have experienced it.