To preface here, what I'm to share goes beyond what's in the article. That said, I worked for years in a clinical "bone lab" (and at a notable university hospital with respect to this area of research) investigating bone wasting diseases; namely, idiopathic hypercalciuria, and osteopenia & osteoporosis.

Not say that lends me too much authority on the matter, but many of the biological effects of D3 are pretty poorly understood, still. Simplifying a bit, this can be chalked up to the fact that it's roles in molecular networks across the systems are myriad, and of these, many of its (downstream) effects seem -- and, if not, are -- contradictory. (To the initial point, in it's bioactive form, it's classified as a hormone.)

At high dosages, typically what's found it many OTC supplements (5-10K IUs vs 500 IU daily allowance), there is evidence that it facilitates significant bone resorption.

So not only does it not strengthen bones, D3 actively makes it more brittle. Typically, this is linked to inadequate calcium & phosphate minerals to match the excess D3 (and these minerals in excess introduce additional problems of their own).

I didn't cite literature here but it is readily available & abundant (But I can pull some if anyone would like). And, within it, there is a glaring lack of consensus. And hence, why my language is couched in so many conditionals.