I'd like to see the actual paper.
Researchers are allowed to have opinions as long as those opinions do not influence the study in a manner that skews the results or analysis. So, if his study was sound and found a correlation, as you suggest, then there is nothing wrong with the science. That should be very clearly reflected in the experimental design, results, and data analysis.
If in his discussion, or subsequent discussions, he suggests the possibilty that there causative relationship, that does not invalidate the science or make it "bad science". If anything, it's drawing bad conclusions based on good science. There should, however, be discussion along those lines to further the research and encourage an experimental design that can uncover a causative relationship, or not, if at all possible.
As far as research goes, from what I have read so far, the fact that he does show low levels of vitamin D precursors pre-diabetes diagnosis and subsequent development of diabetes in those with low levels of vitamin D precursors is compelling, though not causative. That's certainly more compelling than a lot of reasons I've read for doing a lot of things that are claimed to be "researched-based" in the control of diabetes.