They're implying that some such research has already been done, but wasn't published because of publication bias against negative results. So the data exists,

They're implying that some such research has already been done, but wasn't published because of publication bias against negative results. So the data exists, but is missing from publication. (I suspect it's also possible that more skeptical researchers have been dissuaded from even doing the research, for fear they'd be wasting their time and research budget because after they did the research, it wouldn't get published.)