I see nothing wrong with this. I see something wrong with asserting, ad hoc, that an Si will report more chronic pain, and then looking for evidence to prove it. At least statistical analysis has a certain correlative relevance.
One thing I have noticed in my personal life is that the ISXJ does not complain overtly about chronic pain. So the actual percentage may be under-reported due to dishonest reporting. And in my experience, the ISXJs have a high tolerance for pain. They are aware of it, and they find means to manage it, but it doesn't keep them from pursuing their activities unless it becomes debilitating. It isn't used as an excuse to quit.