The study that Dr. Chen cites is not isolated. Dan Hurley wrote an excellent book called 'Diabetes Rising', and devotes an entire chapter to the subject of Vitamin D and type 1 diabetes (chapter entitled 'Sunshine Hypothesis').
He quotes a professor at Catholic University in the Netherlands named Roger Bouillon, MD, PhD. He recited a quote from Dr. Bouillon in 2002: "There are enough data to conclude that vitamin D supplements can protect beta cells. We ... urge especially the parents of children with a higher risk for type 1 diabetes (e.g., first degree relatives) to adhere to these supplements and to avoid even marginal vitamin D deficiency."
Hurley makes a very strong case in the chapter that there is a major link between the lack of vitamin D and the development of type 1 diabetes. I wish the chapter was online someplace, but I just checked the book out of the library. I refer to the book so often, I may have to buy a copy.