An abstract is located
here, though it doesn't answer questions about ethnicity (darker skin needs more sunlight for vitamin D production), how the participants were dressed, etc. "This implies that the common clinical recommendation to allow sun exposure to the hands and face for 15 minutes may not ensure vitamin D sufficiency" in the article itself implies that the participants were often fully dressed. Hence, the recommendation to exercise outside, obviously with more skin exposed than if one were, say, dressed for business. (For most people, 15 minutes of almost full exposure won't cause skin damage, yet it will lead to much more vitamin D production than just exposing the hands and face.)
Supplemental Vitamin D3 can boost vitamin D levels, but it's not very effective in everyone. Not everyone consumes the foods with fortified vitamin D, which probably is less effective anyway. Never using sunscreen in Canada might not be so bad for the skin, but closer to the equator, that would be foolish.
