Herein may lie our fundamental disagreement. To clarify though, what I said was that they couldn't write the book based on their own observations. ISTJs excel at condensing and relaying studies which are already out there, however. We have an ISTJ MBTI author on our very forum, after all.
Capacity. Does the INTP have the capacity to do munane data entry for hours and hour and hours? Yes. Would it be likely that you would ever find an INTP doing that? No, it's contrary to their natural tendencies or inclinations.
The same is true with the ISTJ. They have the capacity write the book based on their own observations. But it would take a lot of effort and they probably wouldn't be very happy about it.