[Strilanc]

First, waves still pass through each other when destructively interfering. They temporarily cancel out each others' amplitudes, but not each others' change-in-amplitude, so they continue to propagate.

Second, XOR is not a universal gate (even when combined with NOT). XOR can only sum things together (mod 2). For universal computation you need to be able to multiply (e.g. with an AND gate or a Toffoli gate).