Actually, an ad hominem is when you try to discredit an argument somebody made by attacking that person.

The example you cite seems closer to attacking a person based on their arguments being (perceived or claimed to be) bad. If you say "embarrassing an XY grad would make such a stupid argument" it will only discredit the argument if I believe XY graduates are stupid (I guess the $publication example aims at this). Meanwhile, if I don't see why the argument is bad and don't have a bad opinion of XY, your statement is entirely unconvincing.