But that "we" includes people that didn't venture out of the tropics, who presumably are impacted the same way by UVB, which if true would refute the hypothesis.
At a glance the study doesn't seem to investigate that at all.
The authors are from Israel, so I guess the participants are Israelis too. That's one of the more ethnically diverse places, but only 12% of Israelis are of African origin. Of course not all of Africa is in the tropics, but there are a couple of Asian Israelis who are from the tropics, so let's estimate it at about 12% with mostly tropical ancestors. That might just vanish within the noise of the data unless you specifically look at the correlation to ethnicity.
That's why I said "presumably". If it turns out that a group of people who've lived in the tropics for the last 50k years do not respond to UVB in the same way, then the hypothesis isn't refuted (and indeed it is validated).
