A fixed blade is a stronger design then a folder. But many of us can't carry a fixed blade in day to day life. The point of a small survival kit is to keep you alive for the short period of time it takes to find help and be rescued. If we knew that today you well be in a survival situation we wold pack a ruck sack with all we would need. But bad things happen when you dont expect them and when they do whats in your pockets may be the difference between life and death.
I don't have a BK9 in my pocket but I do have one or two of my trusty SAK's and the odds are that if something bad happens to me I well have one or more of my trusty SAK'S in my pocket.
We can debate fixed blade vs folder all day long but when it comes down to it if you only have a Classic SD on you and you get stuck then be glade you at least have that.