My sense is that Slate mischaracterized the report with item #5. That is, one might read that and think that the report suggests mass shootings are not A problem. The report instead sees mass shootings as a large problem, but one that is proportionally smaller than the overall problem with firearm violence.
For example, the report states
I think that Slate should have been more surprised by the efforts that have been undertaken to limit the research needed to answer these questions. The report states:
My sense is that most people don’t realize that researchers have been proactively and explicitly prevented from studying this topic.