The authors are testing the idea the levels of vitamin D in blood cause differences in rheumatoid arthritis(RA).

They compare relations between genetic variant of both (Vit D and RA) and show no or very weak causal association. Therefore there us no evidence that Vit D is a treatment for RA. More likely an effect of RA.

It's not so much the language as the horrible layout - with all the navigation at the left leaving the text so small.

The reported lower vitamin D levels in more severe RA are more likely to be consequence than cause. Our data do not support vitamin D supplementation as a direct therapeutic intervention to modify RA outcome,