I'm curious why it isn't listed under distributions. Is it because they have made modifications to the drupal core?
Press-flow generally takes the established features of the next version of Drupal and back-ports it to the current version.
For example Pressflow-6 Takes the features(enhancements and performance improvements) from Drupal-7 and back-ports it Drupal-6. These changes generally include changing the core to considerable amount.
The only reason I think of why it is not included is that
Since it makes changes to the core it doesn't make sense to host it on the Drupal.org under project.