I don't understand this at all. His final conclusion did not feel like a causal statement to me as it relates to the premise he had, but a phenomenon/general claim he is describing using the correlation found in the premise as justification. "Healthy dietary behaviour" to me is not the same thing as having "lower fat calories", and so I don't understand how the weakness of the argument is that the author infers a cause from a correlation when the statement in the premise and the statement in the conclusion are not one and the same.