This is one of those classic "sounds dumb / doesn't play well on TV but is actually smarter than most of the other people babbling about it" things. Nassim Taleb has written for example about how maddening it is to watch world-class economists who are also just sort of awkward and a little nerdy go on TV and "lose" to blowhards who don't actually know what the hell they're talking about but appear confident and look good on camera. Thankfully in Rumsfeld's case I think as time has gone on it's become a pretty respected statement about risk even if people still occasionally find the phrasing a bit amusing.