As someone who has had rheumatoid arthritis for just over 12 years (I was diagnosed three weeks after my 9th birthday), I couldn't disagree with this any more. I had never been a tea drinker in my life, no matter how much it was recommended to help relax me, I always refused. I never liked it so I wouldn't drink it.
However, after I turned 18, I started having severe flare-ups that made it nearly impossible to even hold a spoon. I decided to give tea a try due to being lactose intolerant so hot cocoa at the time was definitely not a good idea, and the tea actually relaxed me a little bit enough to help me feel a bit better.
It didn't entirely fix it, but it has never made it worse since I began drinking it when having severe flare-ups over the past three years.

I'd love to see how the data was even correlated for the study though. Because, as a current college student, the main thing drilled into my brain through my psychology courses is that correlation does not always equal to causation.