Good post, but you're assuming they tested the toothbrush right away. I think it's been sitting around.
Natalee got back to the room after the boys dropped her off, and she startled the local tooth brush fetishist who was in her room, so he had to kill her.
I'm no DNA specialist but I don't think the DNA would've degraded significantly if the toothbrush wasn't used again.
I'm not trying to be crass but maybe Jug used hers one morning thinking it was his.
TAW