He did have the proof to back up what he said - that the treatments were bogus. I.e. there is plenty of evidence that they did not work (more accurately no evidence that they do better than a placebo).
The original judge decided that "bogus" meant that the supplier was dishonestly lying about it too, and that was the libellous claim, and that is the appeal he won.
In any case he probably could have won the argument as he could have shown that the Chiropracters ought to have known about the studies that showed the lack of effect, and if they did not they were negligent, and if they did they were dishonest. This however was a much tougher argument, with room for scope of argument on "dishonest".
Notably the BCA had to issue warnings to members to remove claims from websites and literature as there were many making claims that could not be backed up.This suggests that he probably had a point anyway.
The effect is now that many people will not speak out against treatments without any medical value and dodgy medical claims for fear of being sued - even if they win they lose a few years of their life and earnings.