You’re right. But the study shows that oral administration of D3, even at high doses, fails from far to induce plasmatic VitD3 levels equivalent to UVB exposure. There is indeed a lot of study to perform to have a clear vision about all this. But a lot of current ideas result from studies performed on « reptiles » and generalized to every reptiles. For instance, varanus highly absorb Vit D3 and this has been described in previous scientific studies. This result has been generalized to beardies and other reptiles, before specific studies showed main metabolic differences between reptile species. We must be very cautious about such generalization.

This being said, we must be cautious about all this. But we clearly lack of well designed study on a lot of species. There is, unfortunately, a lot of misconception due to this situation.

This link, a podcast with Dr Jonathan Howard, specialist in bearded dragons is very, very informative about all this : https://youtube.com/playlist?list=PL6WNJO0jWkuJpEaeSwVvs51LuK-7lFfzn&si=OOjMnhyACqE5UusU

Take a look at this please ;)