That seems very right. It doesn't matter if covid-19 causes/worsens deficiency or deficiency causes severe covid-19, or both. If you're already deficient the outcome will be that it gets even worse for a deficient person.

I was more making my initial comment thinking in the opposite direction: that people who are not deficient shouldn't feel overly secure (and take more risks) just because there is a correlation - with unclear causation.