If you would have actually read the report, you would have known the attacks are not specifically device or mobile OS bound. They simply say that iOS leaves behind more useable traces.

From paragraph 10:

  Much of the targeting outlined in this report involves Pegasus attacks targeting iOS devices. It is important to note that this does not necessarily reflect the relative security of iOS devices compared to Android devices, or other operating systems and phone manufacturers. In Amnesty International’s experience there are significantly more forensic traces accessible to investigators on Apple iOS devices than on stock Android devices, therefore our methodology is focused on the former. As a result, most recent cases of confirmed Pegasus infections have involved iPhones.