I got the same impression when seeing Kimbo in the back and then walking to the cage. He looked very, very nervous, not the usual focus he displays. Of course it's all speculation, but to me he looked all wrong.
Ken couldn't count on that flash knockdown, he would have been busy swinging for the fences and eating combinatons. He would have been destroyed. He's just too old and too broken down. Hell, he couldn't even make it to the cage, without getting injured on the way.