The memory of the body...
I think what might help understand that concept is to go back to remember what are some of CLAMP's most cherished philosophy/principles for life.
Of all the philosophies they have proposed, the one most closely linked to this concept would have been the same concept that was being dealt with in CLAMP's debut work- RG Veda and their most renowned tragedy X.
First of all, every action counts. This has been reiterated by Yuuko over and over.
Second, both stories talked about how the interaction of the people can alter the future. The love between Asura and Yasha was an example, and in X, there's always the motif of the grinding gears.
I think CLAMP do believe in fate with the Hitsuen idea, but they probably do not believe that fate is an absolute power, but rather, the series of interactions amongst the livings (hence, the gears in X).
Sakura's memory of the body... probably meant the role which she plays in each of the world she travels since she wakes up. Even a single action CAN affect the future, doesn't it?
Fei Wong, by obtaining the power to manipulate every dimension, would find it easier if there is just ONE PERSON who has played some kind of role in each of the worlds.