This conclusion (10x higher mortality if deficient in Vitamin D) is rather significant. How trustworthy is this source?
Group B is going to have a vitamin D deficiency compared to group A. Group A is also much more likely to survive Covid. However, this wouldn't give you any evidence that vitamin D is the cause of better survival rates. You should assume that it might just be an indicator of better health over all unless you can find some way to control for all the other health factors.