I have to disagree with what you have as the correct answers for this question. The question states "According to Microsoft SQL Server Books online, what principles are not adhered to in this design?" and in the link you provided it does not list "Multiple identifier columns" as a problem.

In addition the OrderNo column is NOT an identifier because it does not have a constraint defined on it to prevent duplicate values, or at least the DDL shown doesn't show a constraint. (Yes I know you would have to turn identity insert on for the table, but that can be done, so that field just defaults to be a sequentially numbered field, but it could contain duplicate values.)