While XPoint does work through a phase change, its behavior is different from PCM. Since PCM works through the relative resistance of the amorphous/crystalline material itself, it is slower to write (you have to get the state within the tolerance) and slower to read. It also suffers from wear out as the ability to be sufficiently exact breaks down over time (I am simplifying of course). On the plus side, it allows for multi-bit cells based on intermediate states.
XPoint, the phase change is just a mechanism to allow or block the wire through the cell. This means that reading and writing are much faster, and there is less wear out risk, but there are no intermediate states for multi-bit cells.
Since the behavior is different, I'm sure the marketing guy did his job and told the engineers to give it a new name and avoid the words phase change. In this case it is the right decision as knowledge of PCM would lead you away from understanding the behavior of these new cells.