The author doesn’t have to make such an aggressive assumption. His conclusion is only that some cases of high blood pressure are not caused by stress—not that no cases are ever caused by stress.

The basis of the researcher’s argument is that some cases of high blood pressure were treated with medicine, rather than stress reduction. He therefore has no need to assume that stress reduction can be an effective treatment; that, if anything, weakens his conclusion.

If this is negated, reduced stress does reduce a person’s responsiveness to blood pressure medicine. But this doesn’t contradict the researcher’s argument: the medicine could still be effective, even if it’s less so than it otherwise would be.

The researcher’s premise is that illnesses caused by stress are only treatable by stress reduction. If high blood pressure is treatable by both stress reduction and medicine, it’s consistent with his conclusion that it isn’t always caused by stress.

If this is negated, medicine used to treat high blood pressure does itself reduce stress. If so, this contradicts the researcher’s argument: maybe the medicine works by reducing stress. We would have no reason to believe some cases of high blood pressure aren’t caused by stress.