"(A)Responses need not occur in the same order as requests - (B)But request order establishes the total order for the system"
Under A, in what way is B guaranteed?
Suppose P1 requests X and then P2 requests Y. If X!=Y, we can send P2 a response before sending P1 a response. This is possible because the request order does not impose any restrictions on the response order.
Suppose P1 writes X and then P2 requests X. We need to handle P1's write before P2's read. This is because the request order imposes restrictions on the response order.
Thus, we can guarantee B in spite of A.