Postgres-XC isn't PostgreSQL. Entirely different product.
Anyone can add pages to the wiki, and there's lots of information there about things that aren't postgresql, Postgres-XC is just one of those.
I think "entirely different product" is not really accurate. It isn't just a fork, but a closely related one, which continues to work closely with the community. That's different from Greenplum these days. At any rate it is far less an "entirely different product" than Illustra was, or EDB's Postgres Plus, or countless others.
Why should a user care? Because it means that there is better compatibility than there would be for a typical fork. It is still a much more complex product, and for a much more complex niche. However there will be areas where they are not the same, particularly when it comes to performance.