::[Update] In reading over Professor Salmon's Introduction to Logic and Critical Reasoning, I have found a passage which appears quite clearly to support my position that UD's example is an ad hominem (as opposed to an argument from authority).  Professor Salmon analyzes a dispute between two scientists, using it to give practical examples of informal fallacies.  Consider the following example: 
 :::"Dr. Alvarez responds by saying that he considers Dr. Clemens inept at interpreting sedimentary rock strata and that his criticisms can be dismissed on grounds of general incompetence, a charge Dr. Clemens rejects." 
 :: Professor Salmon classifies this example as an "abusive ad hominem," (p. 125) and it appears to be of the same form as UD's example: i.e. (1) X says Y. (2) X is incompetent. (3) Therefore, ~Y.  
 :: I believe that this really ought to put the matter to rest, unless someone can find a source supporting Mr. Richfield's position (in which case it might be helpful to include both views, while noting the dispute).