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Does this mean if you’re D deficient you’re more likely to have arthritis?

I don't believe the researchers are clear as to cause or effect. They do see a correlation.

In this case they indicate a desire to test whether increased vitamin D supplementation helps correct RA. Otherwise, I guess, they would have to follow peoples' vitamin D levels for decades to get a good idea if low levels of vitamin D cause RA. And just a good idea because other factors would have to be considered.

IOW, I don't know.

"Does this mean if you’re D deficient you’re more likely to have arthritis?"

Yes, it certainly suggests this, however, we don't know the direction of causality. Is D deficiency driving the autoimmune response or the other way round? They tried to see if increasing D would have an impact, but 800 to 1000 IU seems low by current research findings. Some experts are recommending daily doses in the 5000-10000 range, and even adding some starting booster shots. The recommended intake for pregnant women was just raised 10-fold, from 400IU a day to 4,000IU. The old 400IU RDA was based on very old research that showed this level prevents rickets. As a point of reference, 20 minutes in the sun can produce 10,000IU in the body. Vitamin D, which has almost hormonal like properties in the body, has far reaching impact on many diseases and health. It seems every week, another study shows the power of Vitamin D.