Dude. Let me break it down for you.

Re-read: Suppose there is one school-age child, somewhere in the world, whose behavior is affected by artificial food colorings, and who is claimed to be sensitive to food coloring. Then the statement, "artificial food colorings do not affect the behavior of school-age children who are claimed to be sensitive to these agents," is false.

Claim A: Artificial food colorings do not affect the behavior of school-age children who are claimed to be sensitive to these agents.

Hypothesized fact B: There is one school-age child, somewhere in the world, whose behavior is affected by artificial food colorings, and who is claimed to be sensitive to food coloring.

You just said that claim A is true given fact B. I don't need to read the article you linked to to know that's wrong. We aren't talking about how people use English colloquially. We are talking about how they use it making logical claims. And I know, from many years of experience, that when a doctor reads a study that concludes "X does not cause Y", they interpret it as meaning "X never ever ever causes Y".

Doesn't matter anyway, since all they proved was "There exists AT LEAST ONE child not affected by food coloring." That's not even close to what they claimed.