If you feel strongly about it, you could always read the study properly and give us a summary of its flaws (as would I if I cared enough). Scientific articles are usually quite amenable to investigation.
Saying you can ignore inconvenient studies because academics don't like Trump puts you on a pretty weak footing in my eyes.
I have no desire to spend my time doing such. I've given you my logic on the subject.
Any illegal that is here and commits a crime, that is a crime that should not have happened. If they are not causing crime to increase, they are preventing it from decreasing.
I tend to think of crime rate as per head of population, per unit time. For example murders per 1 million people per year. From that point of view if you introduce new people whose rate of committing crimes is less than the established population, they are decreasing the crime rate.
You seem to be thinking of a raw sort of crime rate per unit time so that more people = more crime basically no matter what people you introduce. If you don't divide by a standard unit of population, you'll also find that getting rid of any class of people should reduce the crime rate by reducing the population.
Anyway, as you've said, your opinion on the matter is clear.
If one illegal commits a crime, that's one crime that happened which should not have. Therefore the crime rate was increased.
We all know the academics are heavily biased against Trump, I would not be surprised if that carried into this study.