But we're talking about this guy named Stewart Baker, who just so happens to have authority as a senior officer of the executive branch of the United States Government.
This is also a type of authority. It is not the relevant type of authority when discussing whether or not a statement is an ad hominem.
Authority: 3. • a person with extensive or specialized knowledge about a subject; an expert: she was an authority on the stock market.
From the Oxford American Dictionary
It's this meaning of the word authority used in Appeal to Authority: http://en.wikipedia.org/wiki/Argument_from_authority