They are non-overlapping memory areas: "Note that in our case the source and destination memcpy() regions are completely disjoint." (2nd para) And the array is

They are non-overlapping memory areas: "Note that in our case the source and destination memcpy() regions are completely disjoint." (2nd para) And the array is not left in an inconsistent state. They are doing the equivalent of memcpy (a, b, n); memcpy (b, a, n); and a different thread running at the same time observes incorrect data in the supposedly unaltered `b[]` during the second memcpy.