> But talking about an investigation into FIFA is somehow on-topic and relevant to HackerNews?!

To the extent that the FIFA discussion is inappropriate for HN, the correct response is to flag the article, not post things off-topic to the article in the thread.

Whether the FIFA-related article is appropriate to HN is completely irrelevant, in any case, to discussion of whether the attack on Clinton is itself on-topic to the thread discussing the FIFA article.