Could you cite that claim from the paper?
> An adult in a bathing suit exposed to 1 minimal erythemal dose of ultraviolet radiation (a slight pinkness to the skin 24 h after exposure) was found to be equivalent to ingesting between 10,000 and 25,000 IU of vitamin D (Fig. 6).
Doesn't say 30 minutes, but it may be 30 minutes depending on your skin colour and the local strength of the sun.
So the paper may be well researched or whatever, but the interpretation of it is questionable.
From the abstract:
> The safe upper limit for children can easily be increased to 2,000 IU of vitamin D/day, and for adults, up to 10,000 IU of vitamin D/day has been shown to be safe. The goal of this chapter is to give a broad perspective about vitamin D and to introduce the reader to the vitamin D deficiency pandemic and its insidious consequences on health that will be reviewed in more detail in the ensuing chapters
The full article is available on researchgate[1]. Direct link to PDF [2].
[1] https://www.researchgate.net/publication/226676251_Vitamin_D...
[2] https://www.researchgate.net/profile/Michael-Holick/publicat...
EDIT: I just looked up the author, Michael F. Holick. Apparently he is one the people who identified calcitriol in 1971. I know appeal to authority doesn't prove anything, but it might be prudent to at least consider his findings.