Although this is a notable finding it's important to remember that it doesn't lead to the conclusion that supplementing vitamin D will be very beneficial here. When vitamin D is produced in the body naturally via exposure to sunlight there's a huge amount of other things that occur as well, and we don't have a ton of good science to back up if curing a vitamin D deficiency via supplementation of pure vitamin D is really nearly the same as getting it naturally, which is a common problem for supplementing things and nutritional science and epidemiology in general.

The extent that covid harms those that are metabolically unfit in many different ways much more than those that are perfectly healthy is still pretty under-discussed imo though, and this is yet another good data point in favor of it, even if Vitamin D is much more of a different proxy than some of the other large correlations we see with covid mortality.