If the service is paid employment as an LPN, then it isn't volunteer service.
You are misreporting this paper. It was clinical experience, not clinical volunteering. Those with clincial experience include former health professionals such as nurses and EMTs who move on to medical school and who may be less succesful academically in medical school compared with students who did not have clinical experience. Also, consider that this was done at Uniformed Services which may be attracting two different groups that are much different than the students at other med schools: graduates of the service academies who are unlikely to have obtained clinical experience but to highly intelligent, have a strong academic work ethic, and have an undergrad GPA that was not necessarily very strong (because of the rigor of the service academies). The other group would be former military medics who have extensive clinical experience but who may not have attended schools as rigorous as the military academies or top academic institutions in the US (3.85 from service academy is not equal to 3.85 from mostly community college pre-req classes). So, I could see how these results may have come out as they did.
We know that those who major in the health professions and who are admitted to med school have the lowest average GPA and MCAT. It may well be that they get in based on clinical experience. We may now see that controlling for undergrad GPA, the clinical experience is not going to close the gap in med school GPA and step 1 exams compared with more traditional students.