Another way to phrase of your point: If you consider it possible they might have told you "I have two children and (at least) one of them is a girl" instead of

Another way to phrase of your point: If you consider it possible they might have told you "I have two children and (at least) one of them is a girl" instead of the statement about a boy (were they to have both a boy and a girl), the reasoning given in the article is wrong.