If I tell you that not all apples are delicious, is that the same as saying there isn't a single apple that is delicious ("none" of the apples are delicious)? How many apples are we 100% sure are not delicious when "not all" of them are?
You are totally right. I postpone taking the LSAT myself when I was diagnosed with Hodgkin's lymphoma, as I realized that you need to be 100% focused and prepared. Best of luck for everything!