This doesn't pass the smell test for me.
Note the incremental difference in CRP levels measured - 0.06 mg/dL, or 0.6 mg/L CRP per 10 ng/mL 25(OH)D above 21 ng/mL.
Depending on the resolution of the test performed, a change like this might not even be measurable. In other words, the test has to be very sensitive to detect changes like this.
I have had a CRP test done at least twice while taking vitamin D, and on the first test, my 25(OH)D was 66 ng/mL. Guess what my CRP level was? Undetectable low (below 0.2 -- that was all the result could show)mg/L).
A more recent test at a different lab showed it at 0.1 mg/L, and that was after the level had dropped to 21 ng/mL and I started correcting by supplementing 5000 IU vitamin D3 a day. I don't know what the resolution for the CRP test was, but I would be willing to bet that 0.1 is within the margin of error for the test. I had miserable skeletal pain at 21 ng/mL of 25(OH)D, and I wasn't making it up. We ordered the test because of the pain.
Of course, everybody is different, but my point here is this: everybody is different! If I don't take 5000 IU a day, my levels will not don't go above 30 ng/mL, and I will have symptoms.
I have to second the other posters. What else were these people doing? Were they nutritionally replete? Getting enough calcium, magnesium, vitamin K, vitamin A? All unlikely.
I would still be more comfortable somewhere between 40 and 60 ng/mL. The author of the study is right about one thing, though: knowing is better than guessing. Get tested.
EDIT: I just realized that I got my units mixed up, and I have fixed them.