Don't get me wrong, personally I think it's very valid to bring up and important to keep in mind while reading their article. But it seems like a double standard to me.
In the legal system it's perfectly acceptable to bring up such an ad hominem argument: motive.
No one should be convicted on motive alone, however it does frame the interpretation of facts in a certain way.
In this particular case we have a seemingly sensationalist article, along with some suspect data collection, and then a large jump from correlation -> causation about nuclear fallout. Once it is seen that the authors have a vested interest in damning nuclear power, it's much easier to draw conclusions about what's going on or to increase scrutiny considering the impartial nature of the researchers.
and the relevant HN discussion: http://news.ycombinator.com/item?id=2045120
Even if it's Stalin saying "Murder is wrong, because kills someone", it doesn't matter that he himself was a mass murderer. It would make him a hypocrite, and a terrible person, but his logic still stands by himself.
On an unrelated note, I liked your (unintentional?) subtle humor of pointing out who wrote the article on ad hominem.
For example, if a senator wrote an article saying senators' salaries should be increased, one could respond:
Of course he would say that. He's a senator.
Not ad hominem: "I'm a godlike scientist and I'm certain X is true. Trust me." -> "No, he's a poopie head. Don't trust him."
In this case, the only reason we believe the speaker is because we believe him to be a godlike scientist. If he is not, we have little reason to believe him.
Ad hominem: "Here is a study. The methodology and data are available. It claims X." -> "Don't trust him, he's a poopie head."
In this case, the poopie-headed nature of the speaker is irrelevant. His methodology is either valid or invalid. The ad hominem attack is a logical fallacy in this case.