As you can see it's hardly a rule - more a statement - but that's ok.
Another renowned documentary film maker, Joe Berlinger, replies (in one phrasing): "Not every filmmaker can or wants to be funny, and not every subject lends itself to humor".
Who is right? If you've been here before you know my stand; that it is probably better to fight adversary to humor than making a non-humorus film - even on a very important topic.
But how can you treat a serious problem with un-seriousness? Well, it's up to every film maker, of course, but I think it's really the other way around: Taking a humorous look at ANYTHING only shows the utmost respect for the people involved. And if you feel otherwise, if you get offended, if you feel that the humourus approach is despicable - who is the ones having the real problems? You or the funny guy?