I would never argue against vegetation. I have a garden. It's very small. But better that than no garden at all. Still, I think this study reveals, at best, some correlation, but nothing causative, i.e., a deterrent. I know this city pretty well, and I bet I could take the authors to some vegetative sections of town that would give pause before a stroll. That's why you're unlikely to see many — or any — people there at night. Parks may have a low crime rate just because people only go to them — and at the times when — they are well-lit and well-populated.
The environmentalist sermonette toward the end didn't help.