It seems to me that what the author is talking about is how undisciplined thought can make worse any physical pain condition. He is not denying the reality of physical pain.
Anybody with, say, arthritis will confirm that arthritic pain moves from the knees to the ankles, from the dorsal vertebras to the lumbar region without much pulling or pushing from the part of the sufferer. Physical pain can have a shape that ows nothing to imagination. It can be in the shape of a stomach pain, a pain circumscribed to the area of the stomach and digestive track. Nothing imaginary or mental here: the pain is factually referred to the part of the body that suffers. And then the mental, secondary, reactions to pain under the form of remembrance, anticipation, worry or depression.
Where to draw the line? At one end physical pain can be created by the psyche as with psychosomatic ailments. At the other end, pain unrelated with psyche as with genetic or degenerative diseases. I feel the author is right to say one should'nt superimpose one's ignorance onto the pain.
When I ask the question is the mind in the body or is the body in the mind, where would the answer be likely to come from: from the mind, if it could answer. But the question is so baffling it silences the mind. At that moment there is only pain and no mind trying to deal with it. In this respect animals have an advantage over us humans. A cat suffering from severe kidney pain does'nt move or maul. It does'nt lose one ounce of its energy battling the pain.